Two Stage Amplifier Design using PSpice

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HYBRID MODEL PI

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HYBRID MODEL PI PARAMETERS • Parasitic Resistances • rb = rb’b = ohmic resistance – voltage drop in base region

caused by transverse flow of majority carriers, 50 ≤ rb ≤ 500

• rc = rce = collector emitter resistance – change in Ic due to change in Vc, 20 ≤ rc ≤ 500

• rex = emitter lead resistance – important if IC very large, 1 ≤ rex ≤ 3

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HYBRID MODEL PI PARAMETERS • Parasitic Capacitances • Cje0 = Base-emitter junction (depletion layer) capacitance,

0.1pF ≤ Cje0 ≤ 1pF • Cµ0 = Base-collector junction capacitance, 0.2pF ≤ Cµ0 ≤

1pF • Ccs0 = Collector-substrate capacitance, 1pF ≤ Ccs0 ≤ 3pF • Cje = 2Cje0 (typical) • ψ0 =.55V (typical) • τF = Forward transit time of minority carriers, average of

lifetime of holes and electrons, 0ps ≤ τF ≤ 530ps

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HYBRID MODEL PI PARAMETERS • rπ = rb’e = dynamic emitter resistance – magnitude varies to

give correct low frequency value of Vb’e for Ib • rµ = rb’c = collector base resistance – accounts for change in

recombination component of Ib due to change in Vc which causes a change in base storage

• cπ = Cb’e = dynamic emitter capacitance – due to Vb’e stored charge

• cµ = Cb’c = collector base transistion capacitance (CTC) plus Diffusion capacitance (Cd) due to base width modulation

• gmVπ = gmVb’e = Ic – equivalent current generator

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Hybrid Pi Relationships

Cm

T

T

Cm

C B

I g = V k T V = = 26mV @ 300 K

q I g =

26mV (26mV) ( ) 26mV r = =

I Iπ

°

β

β = gm rπ

πc m π

π

β v i = = g vrM2-3 Electronics

Hybrid Pi Relationships

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Design of a Two Stage Amplifier

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Two Stage Amplifier Design Specifications Design a two stage common emitter amplifier with partial emitter bypass for the following specifications: VCC = 20V VE = .1VCC

RE1A = .25RE1 VC1 = .6VCC IC1 = 2mA RE2A = .4RE2 VC2 = .55VCC IC2 = 2.5mA R2 = .1βRE1 R4 = .1βRE2 RL = 10kΩ fCL1 = 16Hz fCL2 = 13Hz fCL3 = 12Hz fCL4 = 67Hz fCL5 = 8Hz For both stages: β = 140 τCB = 150ps VA = 100V Cµ ≈ 8pF fT = 150MHz rb = 19Ω

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Hybrid Pi Model

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Low Critical Frequencies • There is one low critical frequency for each coupling and

bypass capacitor • We start by determining the (Thevenin) impedance seen by

each capacitor • Then we construct a RC high pass filter (output across Z) • We may then calculate the critical frequency by letting |XC| = Z and solving for either fCL or C and fCL = fCL1 + fCL2 + fCL3 + fCL4 + fCL5

CL

CL

1 f = 2 π Z C

1 C = 2 π f Z

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Hybrid Pi Model Input First Stage

( )IN1 1 2 b1 π1 E1AZ = R //R // r + r + (β + 1)R

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Hybrid Pi Model Output First Stage

( )O1 C1 O1 E1AZ = R // r + R

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Hybrid Pi Model Input Second Stage

[ ]IN2 3 4 b2 π2 E2AZ = R //R // r + r + (β + 1)R

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Hybrid Pi Model Output Second Stage

( )O2 C2 O2 E2AZ = R // r + R

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Hybrid Pi Model Emitter Bypass First Stage

( ) β

1 2 b1 π1 TH_IN1 + E1A E1B

R //R + r + rZ = R // R

+ 1

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Hybrid Pi Model Emitter Bypass Second Stage

[ ] β

3 4 C1 o1 E1A b2 π2 TH_IN2 + E2A E2B

R //R //R //(r + R ) + r + rZ = R // R

+ 1

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fCL1

πCL1

IN1 1

1 f = 2 Z C

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fCL2

( )πCL2

O1 IN2 2

1 f = 2 Z + Z C

Determine the Thevenin Impedance seen by C2

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fCL3

( )πCL3

O2 L 3

1 f = 2 Z + R C

Determine the Thevenin Impedance seen by C3

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fCL4

Determine the Thevenin Impedance seen by CE1

( )πCL4

TH_IN1 4

1 f = 2 Z C

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fCL5

Determine the Thevenin Impedance seen by CE2

( )πCL5

TH_IN2 5

1 f = 2 Z C

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Schematic of Design

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Simulation Profile

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Probe Plot – Y Axis Settings

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Probe Plot – X Axis X Grid Settings

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Frequency Response

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Frequency Response

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