Building Blocks of Integrated- Circuit Amplifiers
Transcript of Building Blocks of Integrated- Circuit Amplifiers
Microelectronic Circuits, Seventh Edition Sedra/Smith Copyright Β© 2015 by Oxford University Press
CHAPTER 8
Building Blocks of Integrated-Circuit Amplifiers
Microelectronic Circuits, Seventh Edition Sedra/Smith Copyright Β© 2015 by Oxford University Press
Figure 8.1 Circuit for a basic MOSFET
constant-current source. For proper operation,
the output terminal, that is, the drain of Q2,
must be connected to a circuit that ensures that
Q2 operates in saturation.
Basic MOSFET current source (current mirror)
Figure 8.2 Basic MOSFET current mirror.
Q2 should be in saturation,πππΊπΊπΊπΊ < πππ‘π‘
πΌπΌππ β πΌπΌπ π π π π π for matched MOSFETS
MOSFET current mirror
Figure 8.2 Basic MOSFET current mirror.
πΌπΌπππΌπΌπ π π π π π
= βππ πΏπΏ 2βππ πΏπΏ 1
Easy to change πΌπΌππ by choosing ππ2
Channel-length modulation (Early) effect:
1) finite output resistance
ππππ2 =πππ΄π΄2πΌπΌππ
=1
ππ2πΌπΌππ
2) (in other words) πΌπΌππ slightly depends on ππππ
πΌπΌππ = βππ πΏπΏ 2βππ πΏπΏ 1
πΌπΌπ π π π π π (1 + ππππβπππΊπΊπΊπΊπππ΄π΄2
)
Microelectronic Circuits, Seventh Edition Sedra/Smith Copyright Β© 2015 by Oxford University Press
Figure 8.4 A current-steering circuit.
Current-steering circuit
Microelectronic Circuits, Seventh Edition Sedra/Smith Copyright Β© 2015 by Oxford University Press
Figure 8.7 The basic BJT current mirror.
BJT current mirror
Figure 8.9 A simple BJT current source.
πΌπΌππ β πΌπΌπ π π π π π for matched BJTs
Microelectronic Circuits, Seventh Edition Sedra/Smith Copyright Β© 2015 by Oxford University Press
Figure 8.10 Generation of a number of constant currents of various magnitudes.
Current steering with BJTs
Microelectronic Circuits, Seventh Edition Sedra/Smith Copyright Β© 2015 by Oxford University Press
BJT current mirror
πΌπΌπ π π π π π = πΌπΌπΆπΆ + 2πΌπΌπΆπΆπ½π½ = πΌπΌπΆπΆ 1 +
2π½π½
Figure 8.8 Analysis of the current mirror
taking into account the finite Ξ² of the BJTs.
πΌπΌπππΌπΌπ π π π π π
=1
1 + β2 π½π½ β 1 β2π½π½
With Early effect:
πΌπΌππ =πΌπΌπ π π π π π
1 + β2 π½π½ 1 +ππππ β πππ΅π΅π π πππ΄π΄2
Output resistance π π ππ = ππππ2 = βπππ΄π΄2 πΌπΌππ
Microelectronic Circuits, Seventh Edition Sedra/Smith Copyright Β© 2015 by Oxford University Press
Figure 8.11 A current mirror with base-current
compensation.
BJT current mirror with base-current compensation
πΌπΌπ π π π π π = πΌπΌππ 1 +2
π½π½ π½π½ + 1
πΌπΌππ β 1 β2π½π½2 πΌπΌπ π π π π π
Instead of πΌπΌππ β 1 β β2 π½π½ πΌπΌπ π π π π π for simple mirror, it is now better:
However, output resistance is still not improved, π π ππ = ππππ2
Figure 8.40 The Wilson bipolar
current mirror: circuit showing
analysis to determine the current
transfer ratio
BJT Wilson mirror
Reduces inaccuracy of πΌπΌππ/πΌπΌπ π π π π π and also improves (increases) π π ππ
πΌπΌππ β 1 β2π½π½2 πΌπΌπ π π π π π
π π ππ = π½π½3 ππππ3/2
(derivations are not trivial)
We will discuss some other improved current mirrors later
Basic IC design philosophy: resistors are expensive (especially large resistances), transistors are cheap.Try to avoid resistors (do as much as possible with transistors).
Idea of active load: replace load resistors with transistors or with transistor-based circuits (current mirrors).
π΄π΄π£π£ = βππππ(π π πΊπΊ β₯ ππππ)
Wish to increase π π πΊπΊ
β
Figure 8.13
Use transistor(s) to create current source πΌπΌ.
Then π΄π΄π£π£ = βπππππ(ππππ2 β₯ πππππ).
output resistanceππ02
Figure 8.15 (a) The CS amplifier with the current-source load implemented with a p-channel MOSFET Q2;
(b) the circuit with Q2 replaced with its large-signal model; and (c) small-signal equivalent circuit of the amplifier.
CS amplifier with PMOS active load
π΄π΄π£π£ = βπππππ(πππππ β₯ ππππ2)
(dc value of the output voltage is not obvious)
Figure 8.16
CS amplifier with current mirror as active load
π΄π΄π£π£ = βππππππ π ππ
π π ππ = πππππ β₯ ππππ2
π΄π΄π£π£ = βπππππ (πππππ β₯ ππ02)
π π ππ = β
(dc value of the output voltage is not obvious)
Figure P8.55
CG amplifier with current mirror as active load
π΄π΄π£π£ = πππππ +1πππππ
πππππ β₯ ππππ2
π π ππ = πππππ β₯ ππππ2
π π ππ β1πππππ
1 +ππππ2πππππ
β πππππ πππππ β₯ ππππ2
Figure 8.45 (a) A source follower biased with a
current mirror Q2βQ3 and with the body terminal
indicated. Note that the source cannot be connected
to the body and thus the body effect should be
taken into account. (b) Equivalent circuit.
Source follower with active load
π΄π΄π£π£ =πππππ
πππππ + β1 πππππ + β1 ππππ3β 1
π π ππ =1πππππ
β₯ πππππ β₯ ππππ3 β1πππππ
π π ππ β β
Actually, the body effect is important, then
π΄π΄π£π£ β1
1 + ππ =1
1 + βπππππππ πππππ
Figure 8.30 (a) A MOS cascode amplifier
with an ideal current-source load
Next subject: MOS cascode
for simplicity MOS cascode: common-source stageloaded with common-gate stage
(similar with BJT: CE loaded with CB)
Idea: 1) increase output resistanceβ increase voltage gain
2) fast circuit because Q1 is loaded with a rather small 1/ππππ2
π΄π΄π£π£ = βπΊπΊπππ π ππ πΊπΊππ β πππππ
π π ππ β ππππ2 πππππ ππππ2
More accurately,
πΊπΊππ =ππππ2 + 1/ππππ2
ππππ2 + β1 ππ0π + β1 ππππ2πππππ
π π ππ = πππππ + ππππ2 + ππππ2 πππππ ππ02
Derivation of output resistance π π ππ for cascode
π£π£ππππ2 = βπππ₯π₯πππππ
π£π£π₯π₯ = πππ₯π₯πππππ + ππππ2 πππ₯π₯ β ππππ2π£π£ππππ2= πππ₯π₯πππππ + ππππ2πππ₯π₯ 1 + ππππ2πππππ= πππ₯π₯[πππππ + ππππ2 1 + ππππ2πππππ ]
π π ππ = πππππ + ππππ2 + ππππ2πππππππππ2 β ππππ2πππππππππ2usual output resistance ππππ2 is increased by ππππ2ππ0π
Similarly, for a BJT cascode (will need later)
π π ππ = (πππππβ₯ ππππ2) + ππππ2 + ππππ2 πππππ β₯ ππππ2 ππππ2
β ππππ2ππππ2ππ02 = π½π½2ππ02
neglect
ππ02 is increased by factor π½π½2
Figure 8.30 (a) A MOS cascode amplifier
with an ideal current-source load
MOS cascode with ideal current source
π΄π΄π£π£ β βππππππ π ππ β βππππππππππ ππππ2ππππ2
π π ππ β ππππ2 πππππ ππππ2
As if two stages of amplification, but faster operation (first transistor loaded with small 1/ππππ2)
Actually, needs a very good current source (with output resistance comparable to π π ππ). Simple current mirror is not good enough (only ππππ), β we need either an improved current mirror (discuss later) or another cascode.
Microelectronic Circuits, Seventh Edition Sedra/Smith Copyright Β© 2015 by Oxford University Press
Figure 8.31 (a) A MOS cascode amplifier
loaded in a simple PMOS current source Q3.
Output resistance is π π ππ β₯ ππππ3.Voltage gain is limited by ππππ3.
Not quite good for voltage gain, but still fast.
MOS cascode with simple PMOS current source
Figure 8.33 A cascode amplifier with
a cascode current-source load.
MOS cascode with cascode current source
π π ππππ = ππππ3ππππ3πππππ
output, π£π£ππ
input
π π ππππ = ππππ2πππππππππ2
π΄π΄π£π£ = βπππππ(π π ππππ β₯ π π ππππ)
If all ππππ are equal and all ππππ are equal,
then π΄π΄π£π£ = β12 ππππππππ 2
Figure 8.35 Double cascoding.
MOS double cascode
Each time increase output resistance
ππππ2 β ππππ2 ππ02 ππ0πβ ππππ3 ππππ3 ππππ2 ππππ2 ππ0π
π΄π΄π£π£ β β ππππππππ 3 if ideal current source
if double-cascode as the current source, then Γ π2
Figure 8.36 The folded cascode.
MOS folded cascode
Equivalent to usual cascode. Avoids βstackingβ (requiring too large voltage).
PMOS common gate load.
Figure 8.37 (a) A BJT cascode amplifier
with an ideal current-source load;
BJT cascode
π π ππ = (πππππβ₯ ππππ2) + ππππ2 + ππππ2 ππππ2 πππππ β₯ ππππ2
β ππππ2ππππ2ππ02 = π½π½2ππ02(as derived earlier)
Common-emitter loaded with common-base
π΄π΄π£π£ β βππππππ π ππ β βππππππ½π½2ππ02
Impossible to double-cascodebecause π π ππ would still be the same
(though can double-cascode with MOSFET in BiCMOS technology)
Needs βgoodβ current source(otherwise significantly less π΄π΄π£π£)
Figure 8.38 A BJT cascode amplifier
with a cascode current source.
BJT cascode with cascode current source
π΄π΄π£π£ = βπππππ (π½π½2ππππ2 β₯ π½π½3ππππ3)
Microelectronic Circuits, Seventh Edition Sedra/Smith Copyright Β© 2015 by Oxford University Press
Figure P8.81
BiCMOS cascode
MOS loaded with BJT: large input resistance of MOS andlarge output resistance of BJT. Also, faster (loaded with ππππ).
π΄π΄π£π£ = βπππππ π½π½2 ππππ2
Next subject: improved current mirrors
Basic BJT current mirror
π π ππ = ππππ2 (not large)
πΌπΌππ β 1 β β2 π½π½ πΌπΌπ π π π π π πΌπΌππ β 1 β β2 π½π½2 πΌπΌπ π π π π π (better)
π π ππ = π½π½3 ππππ3/2 (larger)
Wilson BJT current mirror
Figure 8.41 The Wilson
MOS mirror: (a) circuit;
(c) modified circuit.
Wilson MOS mirror
π π ππ = ππππ3 ππππ3 ππππ2Derivation is not trivial.Result looks similar to cascode, but with different transistor
Extra transistor to balance (so that the same voltages in both branches)
Figure 8.39 A cascode MOS current mirror.
Cascode MOS mirror
π π ππ = ππππ3 ππππ3 ππππ2
Drawback: stacked transistors, βeats upβ more voltage,
ππππ > πππ‘π‘ + 2 ππππππ
(since πππΊπΊ3 = 2(πππ‘π‘ + 2ππππππ)from Q1 and Q4 )
(same drawback for Wilson mirror)
Figure 8.42 The Widlar current source.
Wildar current source
Resistor π π π π decreases πππ΅π΅π π 2 β decreases πΌπΌππ
πππ΅π΅π π π β πππ΅π΅π π 2 = ππππ lnπΌπΌπ π π π π π πΌπΌππ
πΌπΌπππ π π π = ππππ lnπΌπΌπ π π π π π πΌπΌππ
Assume matched transistors
π π ππ = ππ0 [1 + ππππ(π π π π β₯ ππππ)]
(increased output resistance)
Figure 8.44 (a) CCβCE amplifier; (b) CDβCS amplifier; (c) CDβCE amplifier.
Next subject: Some useful transistor pairings
(a) CC-CE: increases π π ππ (due to emitter follower), makes faster (not obvious)
(b) The same with MOS. Faster (no improvement of π π ππ)
(c) The same in BiCMOS: better π π ππ than in (a), better ππππ than in (b)
Figure 8.47 (a) The Darlington
configuration.
Darlington configuration
π½π½ = π½π½π π½π½2
Sziklai pair (compound, complementary Darlington)
π½π½ = π½π½π π½π½2
npn npn pnp
Figure 8.48 (a) A CCβCB amplifier. (b) Another version of the CCβCB circuit with Q2
implemented using a pnp transistor. (c) The MOSFET version of the circuit in (a).
CC-CB (CD-CG) configuration
(a) CC-CB: fast because of CB, while large π π ππ because of the follower
(b) The same with pnp BJT for CB
(c) MOSFET version of (a)