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U N I V E R S I T Y O F P A N G A S I N A NDepartment of Electronics Engineering

ELECTRONICS ENGINEERING 2Prepared by: Engr. Cezar N. Velasco Jr.

Amplifier Operation An amplifier is an electronic circuit which is capable of increasing the signal

magnitude or amplitude without appreciably altering the signal waveform. The biasing of a transistor is pure dc operation. The purpose of biasing is to

establish a Q-point about which variations in current and voltage can occur in response to an ac input signal.

To make the transistor work as an amplifier, it is to be biased to operate in the active region, i.e. base-emitter junction is to be forward biased, while base-collector junction to be reversed biased.

Output of amplifier is linearly proportional to the input.

Four Possible combination of input-output relationship:1. Voltage-controlled Voltage source – the output is a voltage that is a function of

input voltage.2. Current-controlled Current source – the output is a current source that is a

function of input current.3. Voltage-controlled Current source – the output is a current source that is a

function of input voltage.4. Current-controlled Voltage source – the output is a voltage source that is a

function of input current.

General Category of Amplifiers1. Function2. Frequency response

Classification by Function1. Voltage Amplifier – is an amplifier in which the output signal voltage is larger

than the input signal voltage. The amplifier amplifies the voltage of the input signal.

2. Power Amplifier – is an amplifier in which the output signal power is greater than the input signal. This amplifier amplifies the power of the input signal.

Classification by Frequency response1. Audio amplifier – is designed to amplify frequencies between 15 Hz and 20 kHz.

These frequencies are within the audio frequency spectrum.2. RF amplifier – is amplifier designed to amplify frequencies between 10 kHz and

100,000 MHz. No single amplifier can amplify the entire RF range but any amplifier whose frequency band is included in the rf range and is considered RF amplifier.

3. Video amplifier – is an amplifier designed to amplify a band of frequencies from 10 Hz to 6 MHz. This amplifier is also called wide-band amplifier because of such a wide band of frequencies.

General classification of amplifiers based on controlling input parameter and output controlled sources

1. Voltage amplifier – voltage-controlled voltage source. (Op-amps)2. Current amplifier – current-controlled current source. (BJT)3. Transconductance amplifier – voltage-controlled current source. (FET and

vacuum tubes)4. Transimpedance amplifier – current-controlled voltage source. (current-to-

voltage converter)

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Bipolar Junction Transistor Amplifier ConfigurationTo analyze the ac operation of a transistor amplifier, we need an ac equivalent

circuit for a transistor. We need a model for the transistor that stimulates how it behaves when an ac signal is present.

Model – is a combination of circuit elements, properly chosen, that best approximates the actual behavior of a semiconductor device under specific operating conditions.

re = 26mVI D

An ac emitter resistance. It is very important because it determines the voltage gain. The smaller the emitter resistance, the higher the voltage gain.

Bias Confuguration Zi Zo Av Ai Phase Shift

CE Fixed-biasMedium Medium High High 1800

Voltage Divider BiasMedium Medium High High 1800

CE Emitter Unbypassed High Medium Low High 1800

Emitter FollowerHigh Low Low High

In-phase

Common BaseLow Medium High Low In-

phase

Collector FeedbackMedium Medium High High 1800

Collector DC FeedbackHigh Medium High High 1800

Input and Output Characteristic Impedance of Transistor Amplifier Configuration (Approximate, disregard circuit components)

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1. Common Emitter Medium input and medium output impedance

2. Common Collector (emitter follower) High input and low output impedance

3. Common Base Low input and High output impedance

Classes of Amplifiers (Power Amplifiers) The CLASS OF OPERATION of a transistor amplifier is determined by the

percent of time that current flows through the transistor in relation to the input signal.

Amplifiers are categorized depending upon the amount of signal that flows at the output with a given full cycle input signal.

1. Class A The amount of the output signal flow varies for a full 360o of the cycle. The DC bias level (q-point) is usually set at one half of the supply voltage,

so that the output signal can have a maximum output swing The maximum efficiency is 25%, 50% can be obtained using transformer

coupling. Class A operation is the least efficient class of operation, but provides the

best fidelity. Distortion is low. Operate in the active region (linear portion).

2. Class AB The output signal flows for more than 180o but less than 360o. The Q-point is set between class A and class B level. It is at the active region but near cut-off. Efficiency is less than 78.5% Distortion is moderate The complementary class AB amplifier is the most commonly used output

stage. Its circuit operation is similar to that of a class B amplifier, except that the transistors have a slightly positive bias so that a Q-point DC current flows even when the input voltage is zero.

Class AB exhibits an offset output voltage at zero input voltage. However, feedback can be applied to reduce the offset voltage.

A quasi-complementary amplifier uses a composite pnp transistor, which can deliver higher output power than a normal pnp device.

3. Class B The amount of the output signal flow is 180o

The Q-point is set at the cut-off region, but for practical amplifiers, the Q-point is set a little above cut-off to eliminate crossover distortion.

The maximum efficiency is 78.5% A full 360o output can be obtained by using two class B amplifiers, one

amplifiers produce positive portion (0-180o) and the other amplifies the remaining negative portion (180o-360o). And this is called push-pull amplifiers.

Distortion is high. Most commonly used audio power amplifier. Because of B.E. voltage drops, a push-pull amplifier exhibits a dead zone

in the transfer characteristic, which increases distortion of the output

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voltage. The crossover distortion and nonlinearities can be reduces practically to zero by applying feedback.

4. Class C The output signal flows for less than 1800

The Q-point is set below cut-off region. For a common emitter amplifier, the base emitter junction is initially

reverse bias. Efficiency is not important factor, for it is not designed to deliver large

amount of power, but it is the most efficient class of amplifiers. Maximum efficiency is more than 90%. Very high distortion.

5. Class D Amplifier used in applications that are not amplitude sensitive, such as in

pulse circuit in FM system. The efficiency of class D is comparable to class C. Uses transistors as switches where the only modes are switch on and

switch off. It is used in different switching circuits. <180o

<50%

6. Class S Can be used to amplify both AM and FM system.

7. Class E Similar to Class D. <180o

<50%

Analog Design Class A, B, AB, and C. Based on the shape of collector current waveform

for a BJT or the shape of the drain current for a MOSFET in response to a sinusoidal input signal.

In analog amplifier, the input signal to the amplifying devices causes a proportional output current to flow out of the output terminal.

Switching Design Class D and E. In switching amplifiers the output current of the amplifying device is

pulsating.

Common Methods of Amplifier CouplingWhether an amplifier is one of a series in a device or a single stage connected

between two other devices, there must be some way for the signal to enter and leave the amplifier. The process of transferring energy between circuits is known as COUPLING.

1. Direct Coupling The method of coupling that uses the least number of circuit elements

and that is, perhaps, the easiest to understand is direct coupling. In direct coupling the output of one stage is connected directly to the input of the following stage.

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Provides a good frequency response since no frequency-sensitive components (inductors and capacitors) are used. The frequency response of a circuit using direct coupling is affected only by the amplifying device itself.

Direct coupling has several disadvantages, however. The major problem is the power supply requirements for direct-coupled amplifiers. Each succeeding stage requires a higher voltage. The load and voltage divider resistors use a large amount of power and the biasing can become very complicated. In addition, it is difficult to match the impedance from stage to stage with direct coupling.

The direct-coupled amplifier is not very efficient and the losses increase as the number of stages increase. Because of the disadvantages, direct coupling is not used very often.

2. RC Coupling The most commonly used coupling in amplifiers is RC coupling. RC coupling does have a few disadvantages. The resistors use dc power

and so the amplifier has low efficiency. The capacitor tends to limit the low-frequency response of the amplifier and the amplifying device itself limits the high-frequency response.

3. Impedance Coupling Impedance coupling is very similar to RC coupling. The difference is the

use of an impedance device (a coil) to replace the load resistor of the first stage.

Used at high frequencies. Impedance coupling is usually not used for audio amplifiers.

4. Transformer Coupling No capacitor is needed because transformer action couples the signal

between the primary and secondary of Transformer. The inductors that make up the primary and secondary of the transformer

have very little dc resistance, so the efficiency of the amplifiers is very high. Transformer coupling is very often used for the final output (between the final amplifier stage and the output device) because of the impedance-matching qualities of the transformer. The frequency response of transformer-coupled amplifiers is limited by the inductive reactance of the transformer just as it was limited in impedance coupling.

5. Resistive Coupling Connected or coupled by means of resistor

FET AmplifiersField-effect transistor amplifiers provide an excellent voltage gain with the

added feature of high input impedance. They are also considered low-power consumption configurations with good frequency range and minimal size and weight. Both JFET and depletion MOSFET devices can be used to design amplifiers having similar voltage gains. The depletion MOSFET circuit, however, has much higher input impedance than a similar JFET configuration.

Due to the very high input impedance, the input current is generally assumed to be 0 µA and the current gain is an undefined quantity. While the voltage gain of an FET amplifier is generally less than that obtained using a BJT amplifier, the FET amplifier provides much higher input impedance than that of a BJT configuration. Output impedance values are comparable for both BJT and FET circuits.

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1. Common-Drain – a FET amplifier configuration in which the drain is the grounded terminal. Providing unity gain with no inversion.

2. Common-Gate - a FET amplifier configuration in which the gate is the grounded terminal. Providing gain with no inversion.

3. Common-Source - a FET amplifier configuration in which the source is the grounded terminal. The most popular providing an inverted output.

Bias Confuguration Zi Zo Av Phase Shift

Fixed-bias (JFET or MOSFET)High

RG

Medium

RD//rd

Medium

- gm RD//rd

1800

Self-bias bypassed Rs (JFET or MOSFET)

High

RG

Medium

RD//rd

Medium

- gm RD//rd 1800

Zi Zo Av AiSelf-bias Unbypassed Rs (JFET or D-MOSFET) High

RG

Medium

RD

Low

gmRD

1+gmRs+ RD+Rs rd 180o

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Voltage Divider Bias (JFET or D-MOSFET) High

R1//R2

Medium

RD//rd

Medium

- gm RD//rd 180o

Common Gate (JFET or D-MOSFET)Low

Rs//r d+R D1+gmrd

Medium

RD//rd

Medium

gmRD+RDrd

1+ RDrd

In-phase

Source Follower (JFET or D-MOSFET)

High

RG

Low

RD//rd//1gm

Low

gm (rd /¿ Rs)1+gm (rd /¿Rs)

In-phase

Drain Feedback Bias (E-MOSFET)Medium

RF+rd /¿ RD1+gm(rd /¿RD )

Medium

RF//RD//rd

Medium

- gm(RF//RD//rd)1800

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Voltage Divider Bias (E-MOSFET)Medium

R1//R2

Medium

RD//rd

Medium

- gm RD//rd 1800

The transconductance gm is determined by the ratio of the change in drain current associated with a particular change in gate-to-source voltage in the region of interest. The steeper the slope of the ID versus VGS curve, the greater is the level of gm.

The closer the region of interest to the saturation current IDSS, the greater the transconductance.

When VGS is one-half pinch-off value, gm is one-half the maximum value. When ID is one-fourth the saturation current, gm is one-half the value at

saturation.

Multistage AmplifiersThe design requirements of amplifiers normally specify an overall high voltage

gain, high input impedance, and a low output impedance. A single transistor amplifier rarely satisfies these design requirements and multistage are often used to satisfy the design specifications.

1. Cascaded Amplifiers Are stages of amplifiers connected in series, the output of the first stage

provides the input of the next stage. The output of one stage in a capacitively coupled amplifier is connected

to the input of the next stage via capacitor. In direct-coupled amplifiers, The output of one stage directly connected to

the input of the next stage

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2. Cascoded Amplifier A combination or compound transistor configuration utilizing a common-

emitter on the first stage feeding a common base stage. This configuration provides high input impedance and low noise.

3. Darlington Amplifier Super beta transistor meaning it provides a very large current gain. Double emitter follower Beta multiplier Main feature is the composite

transistor acts as a single unit with a current gain that is the product of the current gain of individual transistors.

4. Differential Amplifier A combination or compound transistor configuration designed to respond

to the difference between the two-input voltages Usually the input circuit for most low-noise power amplifiers and

operational amplifiers. Its output voltage is proportional to the difference between its two input

voltages. It has a high voltage gain and is directly coupled to the input voltages and

the load

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Diff-amp Mode of Operation

1. Single-ended input One input is grounded and the signal voltage is applied only to the other

input.

2. Double-ended output Differential input. Two opposite-polarity (out of phase) signals are applied to the inputs.

Each input affects the outputs

3. Common-mode operation One of the most important aspects of operation of a diff-amp can be

seen by considering the common-mode condition where two signal voltages of the same phase, frequency, and amplitude are applied to the two inputs.

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Common-mode rejection Same signals applied to both inputs, the output superimposed and

cancel, the resulting a zero output voltage.

Common-rejection ratio (CMRR) The measure of the ability of amplifiers to reject common-mode signal This suggests that a good measure of the diff-amp’s performance in

rejecting unwanted signals is the ratio of the differential voltage gain to the common-mode gain.

The higher the CMRR, the better. A very high value of CMRR means that the differential gain is high and the common-mode gain is low

Operational Amplifiers Is a modular, multistage device

with differential input and entire assembly composed on a small silicon substrate packaged as an IC.

Was first introduced in 1940s 1948, first op-amp designed using

vacuum tubes. It was used in the analog

computers to perform a mathematical operations Robert J. Widlar at Fairchild brought out the popular 741 integrated circuit

between 1964 to 1968 Basically an excellent high gain dc amplifier.

The input at inverting terminal results in opposite polarity (anti-phase) output.

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The input at non-inverting input terminal results in the same polarity (phase) output.

Differential gain (Ad) Is the gain with which differential amplifies the difference between two input

signalsj

Average common level

Slew Rate The maximum rate at which the output voltage can change with respect to the

input. Represents the fastest response that an op amp can have.

Power Supply rejection ratio Also called power supply sensitivity Defined as the ratio of the changes in input offset voltage due to the change in

supply voltage producing it, keeping other power supply coltage constant.

Features of IC 741 op-amp1. No frequency compensation required.2. Short circuit protection provided.3. Offset voltage null capability.4. Large common mode and differential voltage range.5. No latch up.

BiFET and BiCMOS Some, called BiFET or BiCMOS op amps, combine an FET input stage with

bipolar transistors in the rest of the amphfier, for the advantages of extremely-high input resistance andnegligible input bias current, together with the high gain of bipolar transistors.

Open Loop Configuration of Op-Amp The simplest possible way to use an

op-amp is in the open loop condition

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Slew rate = ΔVΔt



Gain is very large in open loop condition, the output voltage Vo is either at its positive saturation voltage or negative saturation voltage as V1>V2 or V2>V1

respectively. Very small noise voltage present at the input also gets amplified due to its high

open loop gain and op-amp gets saturated. Inability of op-amp to work as a linear small signal amplifier in the open loop

condition. Rarely use in voltage comparator, zero crossing detetor.

Closed Loop Configuration of Op-Amp The utility of op-amp increase

considerably if it is used in a closed loop mode.

Closed loop is possible using feedback (feedback resistor)

The feedback allows to feed some part of the output back to the input.

The closed loop gain is much less than the open loop gain due to the feedback resistance.

Because of the reduced gain, the output is not driven into saturation and the circuit behaves in a linear manner.

Advantages of Negative Feedback1. It reduces the gain and makes it controllable.2. It reduces the possiblity of distortion.3. It increases the bandwidth, frequency range.4. It increases the input resistance of the op-amp.5. It decrease the output resistance of the op-amp.6. It reduces the effects of temperature, power supply on the gain of the circuit.

Linear Applications of Op Amps Output voltage varies linearly with respect to the input voltage. The negative feedback is the base of linear applications. Voltage follower, differential amplifier, instrumentation amplifier, inverting

amplifier and non inverting amplifier.

Nonlinera Applications of Op Amps A feedback is provided from the output to the input terminal, the feedback may

be provided to the inverting input terminal using nonlinear elements like diodes, transistors.

Nonlinear applications are rectifiers, comparators, clampers, limiters, schmitt trigger circuit.

Inverting Amplifier Provides a phase shift of 180o

between input and output. The input impedance is R1.

Gain (Av) = VoVi =−RfR1

Observations:1. The output is inverted with respect to input, which is indicated by minus sign.2. The voltage gain is independent of open loop gain of the op-amp, which is

assumed to be large.

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3. The voltage gain depends on the ratio of the two resistances. Hence selecting Rf and R1, the required value of gain can be easily obtained.

4. If Rf > R1, the gain is greater than 1.5. If Rf < R1, the gain is less than 1.6. If Rf = R1, the gain is unity.

i. Thus the output voltage can be greater than, less than or equal to the input voltage, in magnitude.

7. If the ratio of Rf and R1 is K which is other than one, the circuit is called scale changer while for Rf / R1 = 1 it is called phase inverter.

8. The closed loop gain is denoted as AVF or AVCL i.e. gain with feedback.

Noninverting Amplifier An amplifier which amplifies the input

without producing any phase shift betweeninput and output.

AVF = VoVi

=1+ RfR1

The positive sign indicate that there is no phase shift between input and output. The voltage gain is always greater than 1. The input impedance is extremely large.

Observations:1. The voltage gasin is always greater than 1.2. The voltage gain is positive indicating that for a.c. input, the output and input

are in phase while for d.c. input, the output polarity is same as that of input.3. The voltage gain is independent of open loop gain of op-amp, but depends only

on the two resistance values.4. The desired voltage gain can be obtained by selecting proper values of R f and

R1.

2

K

Voltage Follower The output voltage is equal to the input voltage. It is also called source follower or emitter

follower14



Unity gain amplifier Buffer amplifier Isolation amplifier

Advantages of Voltage Follower1. Very large input resistance, of the order of MΩ.2. Low output impedance, almost zero. Hence it can be used to connect high

impedance source to a low impedance load, as a buffer.3. It has large bandwidth.4. Low output offset error because the input errors are not amplified.

Summer/summing amplifier or Adder Circuit The output level is determined by adding the input signals together (although

the output signal will be of opposite polarity compared to the sum of the input signals).

Depending upon the sign of the output, the summer circuits are classified as inverting and noninverting summer.

Inverting SummerAll the input signals to be added are applied to the invrting input terminal of the op-amp.

Noninverting Summing Amplifier

There is no phase inversion

Difference Amplifier (Subtractor) A difference amplifier will produce an output based on the difference between

the input signals.

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Integrator The output voltage is the integration of the input

voltage. The integator circuit can be obtained without

using active devices like op-amp, tansistors etc., it is called passive integator.

An integrator that uses an active devices like op-amp is called active integrator.

Differentiator Produces the differentiation of the input voltage at its

output The differentiator which does not use any active device

is called passive differentiator. The differentiator that uses active devices is called active differentiator. Produces an output that is proportional to the rate of change of the input

voltage.

Applications of Practical Differentiator1. In the wave shaping circuits to detect the high frequency component in the input

signal.2. As a rate-of-change detector in the FM demodulators.3. The differentiator is avoided in the analog computers.

Comparators Is a type of op-amp circuit that compares two input voltages and produces an

output in either of two states indicating the greater than or less than relationship of the inputs.

Zero-level Detection, application of op-amp used as a comparator is to determine whwn an input voltage exceeds a certain level.

Nonzero-level detector, is to detect positive and negative voltages by connceting a fixed reference voltage source to the inverting input.

Hysteresis A techniques incorporating positive feedback to reduce noise effects. A comparator with hysteresis is sometimes known as Schmitt trigger.

To limit the output voltage levels of a comparator to a value less than that provided by the saturated op-amp, a single zener diode can be used, and this process of limiting the output range is called bounding.

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The Comparator with positive feedback for hysteresis. The upper trigger point and lower trigger point.

Other Applications of Comparator1. Over-temperature Sensing

The circuit consists of a wheatstone bridge with the op-amp used to detect when the bridge is balanced

2. Analog-to-digital conversion A common interfacing process often used when a linear analog system

must provide inputs to a digital system.

Special Purpose Op-amp Circuits1. Instrumentation Amplifier

A differential voltage-gain device that amplifies the difference between the voltages existing at its two input terminals.

Main purpose is to amplify small signals that are riding on large common-mode voltages.

High input impedance, high common-mode rejection, low output offset, and low output impedance.

Made up of three op-amps and several resistors and the voltage gain is set with an external resistor.

AD622, integrated circuit instrumentation amplifier.

2. Isolation Amplifier Provides between input

and output It is used for the protection

of human life or sensitive equipment in those

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applications where hazardous power-line leakage or high-voltage transients are possible.

Consists of two electrically isolated stages. It uses optical coupling or transformer coupling to provide isolation

between the stages. IS0124 is an integrated circuit isolation amplifier Burr-Brown 3656KG is an example of an isolation amplifier that uses

transformer coupling. Require no common grounds between transducer and the processing

circuits where interfacing to sensitive equipment.

3. Operational Transconductance Amplifiers (OTAs) Primarily a voltage-to-current amplifier in

which the output current equals the gain times the input voltage.

Transconductance is the ratio of the output current to the input voltage.

Two OTA Applications: Amplitude Modulator and Schmitt Trigger.

4. Log and Antilog Amplifiers Are used in applications that require compression

of analog input data, linearization of transducers that have exponential outputs, and analog multiplication and division.

A logarithmic amplifier produces an output that is proportional to the logarithm of the input, and the antilog amplifier take the antilog or inverse log of the input.

5. Constant-Current Source Delivers a load current that remains constant when the load resistance

changes.

6. Current-to-voltage converter Converts a variable input current to a proportional output voltage.

7. Voltage-to-current Converter Provide an output load current that is controlled by an input voltage.

8. Peak Detector Op-amp is used as a comparator. Used to detect the peak of the input voltage and store the peak voltage on

a capacitor.

Video Amplifier Used to amplify signals that represent video information. Some amplifiers should be "flat" across a band of frequencies. In other words,

for every frequency within the band, the amplifier should have equal gain (equal response). For frequencies outside the band, the amplifier gain will be much lower.

For other amplifiers, the desired frequency response is different. For example, perhaps the amplifier should have high gain at two frequencies and low gain for all other frequencies. The frequency-response curve for this type of amplifier

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would show two "peaks." In other amplifiers the frequency-response curve will have one peak indicating high gain at one frequency and lower gain at all others.

High-frequency Compensation for Video Amplifiers If the frequency-response range of an audio amplifier must be extended to 6

megahertz (6 MHz) for use as a video amplifier, some means must be found to overcome the limitations of the audio amplifier. As you have seen, the capacitance of an amplifier circuit and the interelectrode capacitance of the transistor (or electronic tube) cause the higher frequency response to be limited.

Since frequency affects capacitive reactance and inductive reactance in opposite ways, and since it is the capacitive reactance that causes the problem with high-frequency response, inductors are added to an amplifier circuit to improve the high-frequency response.

Inductors (coils), when used for high-frequency compensation, are called PEAKING COILS. Peaking coils can be added to a circuit so they are in series with the output signal path or in parallel to the output signal path. Instead of only in series or parallel, a combination of peaking coils in series and parallel with the output signal path can also be used for high-frequency compensation.

Low-frequency Compensation for Video Amplifiers The culprit in low-frequency response is capacitance (or capacitive reactance).

But this time the problem is the coupling capacitor between the stages. This high reactance limits the amount of output signal that is coupled to the

next stage. In addition, the RC network of the coupling capacitor and the signal-developing resistor of the next stage cause a phase shift in the output signal.

Both of these problems (poor low-frequency response and phase shift) can be solved by adding a parallel RC network in series with the load resistor.

Radio-Frequency Amplifier Are used to amplify signals between 10 kilohertz (10 kHz) and 100,000

megahertz (100,000 MHz) (not this entire band of frequencies, but any band of frequencies within these limits). Therefore, any device that uses frequencies between 10 kilohertz and 100,000 megahertz will most likely use an rf amplifier.

The parallel LC circuit used as a frequency-determining network is called a TUNED CIRCUIT. This circuit is "tuned" to give the proper response at the desired frequency by selecting the proper values of inductance and capacitance.

One way of "broadening" the bandpass of a tuned circuit is to use a swamping resistor. This is similar to the use of the swamping resistor that was shown with the series peaking coil in a video amplifier.

One of these problems is the losses that can occur in a transformer at these high frequencies. Another problem is with interelectrode capacitance in the transistor. The process of overcoming these problems is known as COMPENSATION.

The problem of interelectrode capacitance in the transistor of an rf amplifier is solved by NEUTRALIZATION. Neutralization is the process of counteracting or "neutralizing" the effects of interelectrode capacitance.

Magnetic Amplifier The magnetic amplifier does not amplify magnetism, but uses electromagnetism

to amplify a signal. It is a power amplifier with a very limited frequency response. Technically, it falls into the classification of an audio amplifier; but,

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since the frequency response is normally limited to 100 hertz and below, the magnetic amplifier is more correctly called a low-frequency amplifier.

A magnetic amplifier uses a changing inductance to control the power delivered to a load.

The magnetic amplifier has certain advantages over other types of amplifiers. These include (1) high efficiency (up to 90 percent); (2) reliability (long life, freedom from maintenance, reduction of spare parts inventory); (3) ruggedness (shock and vibration resistance, high overload capability, freedom from effects of moisture); and (4) no warm-up time. The magnetic amplifier has no moving parts and can be hermetically sealed within a case similar to the conventional dry-type transformer.

Voltage-feedback Amplifiers (VFAs) With this type of amplifier, the feedback network feeds back a voltage to the

inverting input, and for this reason such op amps are sometimes called voltage-feedback amplifiers. Although this type can have a gain bandwidth product (GB) of several gigahertz, its closed-loop bandwidth is usually limited by stability requirements to lower values.

Current-feedback amplifiers (CFAs) A different circuit topology is used in circuits intended for wider bandwidth at

lower closed loop gain. These are known as current-feedback op amps. In the CFA, the high load impedances lead to a high output voltage for a small input current, and the ratio of output voltage to input current /B is very high. This ratio has dimensions of ohms, and is known as the forward transimpedance, Zf.

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