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Freq Resp Etc

Apr 08, 2018

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Qamar Zahoor
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Physics116A,12/4/06

Draft Rev. 1, 12/12/06

D. Pellett

Amplifier Frequency Response,Feedback, Oscillations;

Op-Amp Block Diagram and

Gain-Bandwidth Product

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Negative Feedback and Voltage Amplifier

(see solutions to Prob. 10.35 for proofs)

AB is called the loop gain.

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Voltage Amplifier: AF dependence on A

B = 1/20, AF = A/(1 + AB) = [A1 + B]1:

A AF

200000 19.998100000 19.99610000 19.961000 19.6500 19.2200 18.2100 16.7

50 14.3

AFmax = 20. If A >> AFmax, AF is insensitive to A. AF is down 3 dB frommaximum when A = 50.

Reduces distortion due to A nonlinearity, allows for variations in amplifiergain from device to device. (What Black wanted back in the 1920s forhis telephone long-distance line amplifiers)

Suppose A is 200000 at low frequency (say 1 Hz) but falling with fre-quency like 1/f at high frequencies due to a built-in low-pass filter withfc = 5 Hz. With feedback, the -3 dB bandwidth would be improved,since AF remains high until A has fallen many orders of magnitude.

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Phase-Shift Oscillator and AB = -1

Phase-Shift Oscillator

Sinusoidal oscillationwhen AB=-1

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Amplifier Low Frequency Limitations

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Amplifier Low Frequency Limitations

(continued)

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Amplifier High Frequency Limits

Model for parallel (shunt) capacitances to ground in amplifiercircuit:

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Amplifier High Frequency Limits

Where are these shunt capacitances?

Model for parallel (shunt) capacitances to ground in amplifiercircuit:

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Common Emitter Amplifier HF Limits

At high frequencies, must consider BE and BC diode capacitances

CBC

CBE

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Shunt Capacitances In Small-Signal AC Models

How to deal with Cc (or Cgd) which connects input and output?

Base-Emitter Diode Capacitance Base-Collector Diode Capacitance

Simple BJT Model at High Frequencies:

Simple HF JFET Model:

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Use Millers Theorem to Split Cc (or Cgd)

Apply to HF BJT model in a

common emitter amplifierwith gain = -A:

Input circuit (be) and output circuit (ce) are now separated11

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Millers Theorem Proof

QED

Given:

Node 2 (write in terms of v2):Node 1(write in terms of v1):

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Common Emitter Amplifier Input Stage

CE Amplifier

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Common Emitter Amplifier Output Stage

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FET HF Model and Analysis

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Input Circuit Upper Corner Frequency for 9.54

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Output Circuit Upper Corner Frequency for 9.54

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MOSFET Amplifier Example

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Effect of CS on Low Frequency Response

Simple HF JFET Model:

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Upper Corner Frequency: Input Stage

Simple HF JFET Model:

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Output Stage Upper Corner Frequency

Simple HF JFET Model:

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Ways to Improve Amplifier HF Response

Reduce Miller effect

Common Base amplifier (see solution to Problem 9.21)

Differential Amp using non-inverting input with invertinginput grounded

Cascode circuit similar to above

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Common Base Detector Amplifier

No Miller effect since cc, cc grounded at base; fast if use fast

BJT (small cc, cc etc.)

*small signalAC model

*

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Can We Understand Amplifier Operation?

This is an amplifier for short pulses of width ~1 ns.

Pulse response is covered in 116B (i.e., beyond the scope ofthis course)but we can understand its operation based on what we have learned sofar in Physics 116A plus basic physics.

We want the output pulse to have a fast risetime (sharp leading edge). If Rs 50 , the input emitter circuit has fc 2 GHz so the BJT delivers a

short current pulse at the collector which follows the input voltage: ic(t)vin(t)/re.

The collector current is integrated: pulse ic(t) dt = Q = Cv to charge thecombined capacitance C = 2 Cc of the input BJT and the first BJT in theDarlington pair, producing the rapidly rising leading edge of v and theoutput pulse.

Integration occurs because the time constant of the BJT collectorcircuit is = RC = 20 k x 2 pF = 40 ns, much longer than the inputpulse width (assume the base current of the Darlington input

transistor is negligible). The pulse height of the output pulse is proportional to the charge of the

input pulse.

The output is thus expected to look like the sketch. The tail can beshortened using a speedup RC network following the emitter followeroutput (not shown).

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Ch k Si l M d l Wi h SPICE

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Check Simple Model With SPICE

Uses MPSH10RF amplifier BJT:cc = 1 pfce = 1.5 pf

SPICE BJT model forMPSH10 is available

from Fairchild website:www.fairchildsemi.com

vin = V(5) (red)vout = V(8) (green)

Good agreementwith simple model

time

voltage

0.0 20.0 40.0 60.0 80.0 100.0

ns

-20.0

-0.0

20.0

40.0

60.0

80.0

100.0

120.0

140.0

mV V(5) V(8)

Fast Pulse Amplifier

********************VEE!4! 0! DC! -12VBB!2! 0! DC! -6RC! 0! 1! 20KQ1! 1! 2! 3! QMPSH10RE1!3! 4! 20KC1! 5! 3! .01uVS! 9! 0! PWL ( 0 0V 1ns 0.05V 2ns 0V )RS! 9! 5! 50Q2! 0! 1! 6! QMPSH10Q3! 0! 6! 7! QMPSH10RE2!7! 4! 500C2! 7! 8! .01uRO! 8! 0! 1KRI! 5! 0! 10K********************.model QMPSH10 NPN(Is=69.28E-18 Xti=3 Eg=1.11 Vaf=100 Bf=308.6 Ne=1.197 Ise=69.28E-18

+ Ikf=22.83m Xtb=1.5 Br=1.11 Nc=2 Isc=0 Ikr=0 Rc=4 Cjc=1.042p Mjc=.2468 Vjc=.75 Fc=.5

+ Cje=1.52p Mje=.3223 Vje=.75 Tr=1.558n Tf=135.8p Itf=.27 Vtf=10 Xtf=30 Rb=10)

********************

.TRAN! .1ns! 100ns

.control

run

plot! V(5) V(8).endcontrol

.END

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http://www.fairchildsemi.com/http://www.fairchildsemi.com/http://www.fairchildsemi.com/
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Other Possibilities to Avoid Miller Effect

Note the common base circuit lurking in both

High Av here

High Av here

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BiFET Op-Amp Simplified Diagram

The differential amplifier and common emitter amplifier use the largeThvenin equivalent AC resistance of a current source along with theinput resistance of the following stage to achieve large gain. See text, Sec.10.2.

Note C1 makes use of the Miller Effect to achieve a large effectivecapacitance for a dominant low-pass filter.

BJTs have identical

characteristics

p-channel JFETs

Current sourceand input

resistance of

next stage play

role of RD or

RC for amplifier

S

D

C

E

IREFVOUT

+VCC

-VEE

C1

Three-stageamplifier:

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Effect of Dominant Low-Pass Filter

Upper corner frequency is multipliedby (loop gain +1)loop gain

Finally,

The maximum phase shift is 90(wont oscillate for resistive B).

The product of gain and bandwidth is constant.

High frequency performance is compromised.28

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Bode Plot: AF and Bandwidth

AF(dB) A(dB) - (AB)(dB)

At low frequenciesin the example above,

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Bode Plot for A702A Op-Amp

No large C1: Could make amplifier with BW of several MHz.

Considerable gain left when phase shift equals 180 degrees at 12.5 MHz.

Not fool-proof: A unity gain voltage follower would oscillate.

No dominant pole:has 3 low-pass filters

in series.Amplifier phase shift>180with significant gainand can oscillatewith a resistivefeedback network.

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BJT CE Large Signal Performance

The maximum output voltage swing is set by BJT cutoff and saturation

Start with the BJT curves of IC vs. VCE for various values of IB, locateQ point

Draw straight line through Q point with slope dIC/dVCE for midband ACsignals (AC Load Line) to determine useful range

For AC, vc = RCic so AC load line slope = ic/vc = 1/RC in this case.Output voltage swing follows AC load line.

3

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CE Amplifier: DC and AC Load Lines

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

ICvs VCE for 2N2222A npn BJT (SPICE simulation)

IC(mA)

VCE (V)

IB = 8 A

IB = 0 A

IB = 4 A

IB = 12 A

IB = 16 A

Q Point

VCCRC + RE

VCC

Max. symmetrical voltage swing when Q point centered on AC load line

At Q, no input, BJT power dissipation p VCEIC = 4 V2 mA = 8 mW

If the Q point is centered, the average power dissipated by the BJT ismax. with no AC input actually less when producing a signal. (Seesec. 9.4 in text for details)

4

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Push-Pull Emitter Follower

Base bias chain keeps both BJTs just at cutoff (or slightly on) at Qpoint No BJT power dissipated if no input signal.

AC input causes one or the other BJT to provide the output.

Maximum average BJT power now 0.1VCEQiC(sat) much more efficientuse of BJTs and power useful for driving low impedance loads at highpower

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