Electronic Troubleshooting

Chapter 5Multistage Amplifiers

Overview

• When more amplification is required than can be supplied by a single stage amp• A second stage is added• Or more stages are added

• Aspects that are covered• Capacitively Coupled Stages• Testing and Troubleshooting• Frequency Response of Cascaded Stages• Using Negative Feedback• Direct Coupled Amplifiers

Overview

• Aspects that are covered• Differential Amplifiers• Emitter Followers• Analysis of a Complete Amplifier System

Two Stage Capacitively Coupled• Characteristics

• Two stages coupled by • Cap – CC • Freq of AC signal under

amplification • High enough to yield

insignificant impedance, XC for CC

• Determining impedance seen by AC signals• DC Power supplies appear

as a ground/common• Equivalent impedance

seen by the output of Q1

Two Stage Capacitively Coupled

• Characteristics

• Gain of the first stage AV1 = rL1/re1

• Gain of the second stage AV2 = rL2/re2

• Total Gain AV(tot) = AV1 x AV2

• Sample Problem• Given: vin = 2mV, AV1 = 40, AV2 = 60• Find voltages at points X and Y on the drawing

2 40 80Xv mV mV

2 80 60 4800 4.8Y X vv v A mV mV V

243111 |||||| QinCLQL rRRRrr

Testing a two-stage amplifier• Check the output of the last stage

• Should have non-distorted signal of appropriate magnitude

• If bad check at the output of each stage• Remove from consideration all properly functioning parts of the

circuit

Troubleshooting Cascade Stages• Test the power supply

voltages – If Good ↓• Insert small AC signal

• Signal Characteristics• Few millivolts• Into first stage

• Follow the testing chart • Page 95 and 96• Quickly sets focus on

defective part of circuit• Divide and fix strategy• Walk through assuming R2 is

an open – 3rd para on page 97

Frequency Response of Cascaded Stages

• Frequency response of amplifiers is limited • At both high and low frequencies around the operating band

• Low Freq limiting

• Attenuation of the output is directly related to

the increasing impedance of CC as the Freq of

the input is decreasing

• As can be seen in the coupling circuit to the right

• XC at lower freq decrease the input signal for

the second stage

• At DC CC is an open

1

2CX fC

Frequency Response of Cascaded Stages

• Frequency response of amplifiers is limited • Low Freq limiting

• A Thevenin equivalent circuit simplifies the analysis

• When XC = RC1 + r in(2nd stage)

» Vin to the second stage is 0.707 of its max

» Power delivered is ½ or -3dB

» The freq at which this happens is the

lower -3dB point or f1

• Example Problem

• See middle of page 98

11 (2 )

1

2 ( )C in ndStage C

fR r C

Frequency Response of Cascaded Stages

• Freq response of amplifiers is limited • High Freq limiting

• Shunting Caps cause high frequency limiting

• Q1 shunted by CCE

• Q2 input shunted by CBE or Cin • The composite shunting Cap for all the coupling circuit wiring

• CS is the parallel combination

• Same for Req

• f2 is the freq at which XC = Req • The half power point or -3dB point

• See example problem • Mid-page on 99

2e

1

2 R q S

fC

Frequency Response of Cascaded Stages

• Amplifier Frequency Response Curve

Distortion Reduction –Negative Feedback• Prime Cause – Large driving signal

• Results of such distortion are illustrated below

• Unequal positive and negative transitions on the output

Distortion Reduction –Negative Feedback

• Prime Cause – Large driving signal• Distortion results from the characteristics of the base-

emitter diode

• The characteristic curve is only linear over a small range

• See the negative transition of Ib • Will yield

» Distorted Ic

» Distorted vO

Distortion Reduction –Negative Feedback

• Negative Feedback• Characteristics

• Supplies fraction of the

output back to the input

• Connection to the emitter

yields negative feed back

• Feedback voltage scaling

» Voltage divider of RE

and RF

Distortion Reduction –Negative Feedback

• Negative Feedback• Effects of negative

feedback• Pre-distorts the output of the

first stage to yield an undistorted output from the second stage

• Will help counter act the distortion generated in the second stage

• IC and collector voltage VQ1

will have the same form

Distortion Reduction –Negative Feedback

• Negative Feedback• Effects of negative feedback

• The more feedback the less distortion• However the more feedback the less gain

• Gain with Feedback• Called Closed Loop Gain• When open loop gain (without feedback) is large compared to closed loop gain

• At least a factor of 10 or more between Open and Closed loop gain

( ) 1Fv ClosedLoop

E

RA

R

Direct Coupled Amplifiers• Characteristics

• Used when low frequency or DC signals are amplified • For example DC signals in a power regulator, or the outputs

of thermocouples

• Simple circuit (typical of Output stages)• Transistor current controlled byVRE Can be changed by:

• Changing RE or VE

EE

E

VI

R

0.7BC E

E

V vI I

R

C CC C CV V I R

Direct Coupled Amplifiers• Simple Amp without Feedback

• Characteristics• AV1 =RC1/re1 , AV2 =RC2/RE2 , AV2 is usually much smaller than AV1

• Problems with circuit• As Q1 temperature increases

» IC increases

» VC(Q1) decreases

» Changes are

amplified by Q2

• Direct coupling

increases temperature

instability

Direct Coupled Amplifiers• Simple Amp with Feedback

• Characteristics• Forward biased on Q1 comes from VRE

• Divided by R1 and R2

• Follow startup• Q1 off VB(Q2) goes positive

• Q2 turns on and VE grows

• VB(Q1) goes positive

• Q1 turns on• IRC1 increases, VB(Q2) decreases

• VB(Q1) reaches 0.7V quickly

• At stability VRE depends on the ratio of R1 & R2

Direct Coupled Amplifiers• Simple Amp with Feedback

• Characteristics• Temperature Stability

• Q1 heats up and IC1 increases

• VC1 and VB2 decreases

• VE decreases, thus VB1

decreases• Q1 then conducts less• Thus VC1 increases

• End result a temperature

change causes less change in output

• CE was added to make a good low frequency Amp• No effect on DC input signals

Direct Coupled Amplifiers• Simple Amp with Feedback

• Characteristics• Temperature Stability

• Q1 heats up and IC1 increases

• VC1 and VB2 decreases

• VE decreases, thus VB1

decreases• Q1 then conducts less• Thus VC1 increases

• End result a temperature

change causes less change in output

• CE was added to make a good low frequency Amp• No effect on DC input signals

Direct Coupled Amplifiers• Real Sample Circuit

• See Figure 5-14 on page 106• Walk-through

• Collector of transistor X101 is direct coupled to Base of X102

• Base of X101 is biased off of R114 through R104 –Temp Stability

• What is the circuit that links the collector of X102 to the emitter of X101?

Differential Amplifiers

• Characteristics• Used to amplify differences between two signals• Can use transistors, Tubes, or Linear ICs

• This chapter deals with the transistor version• Requires two identical transistors and a common emitter

resistor• Both are forward biased

» -15 Supply» Both emitters at -0.7V

» Both IE’s ~ 1mA

» Both collectors = 10V

and VD =0V

Differential Amplifiers• Characteristics

• Temperature stability• Due to identical transistors if the temperature rises both

have the same current increase and VD stays the same• Walk through

• One input has a more positive value» That transistor conducts More, VE increases, VC decreases» The other transistor conducts less and VC Increases

• VD is proportional to the inputs but larger• Example problem on top of page 108

Differential Amplifiers• Characteristics

• Walk through• Impractical to use very high voltage supplies

• Use a constant current source instead» RE can be adjusted for a more accurate current amount

Emitter Followers• Characteristics

• Have unity gain

• Output in phase with Input

• No collector resistor

• Output from emitter

• Provides current gain without

loading the input circuit

• RE = RL for given circuit

• rin = 80 x 1kΩ

Emitter Followers• Actual Circuits

• Load for the DC Amp• VQ1 sees 5K Ω ||30KΩ • The output can drive a 3KΩ

with less than 10% change in output

Complete Amp System• Complete channel of old tape recorder

• Input Section• Mic jack at top Tape heads below• Input amplifier (aka preamp) X101 and X102

• Audio Frequency (AF) amplifier• Another two stage amp after R119, the volume control pot

(top left of part 2 – page113)• Output driver

• Emitter follower, X105, driving the headphone output – top right of part 2 (page 113) • C122 couples AC signal only to headphones• AC output is also rectified and feed to the Play/Record Level

meter.• Record amplifier

• Part 2 (page 113) Mid-page on right

Left Channel of Tape Recorder (Part 1)

Complete Amp System• Complete channel of old tape recorder

• Record amplifier• Single stage amp for recording, X106.• Voltage divider biasing (R138 and R139)• Capacitive input coupling C125.

• High frequency noise roll-off (attenuation)• For Example C102 goes from collector of X101 to base

• It appears to Amp input signal as much larger (value multiplied by gain of that stage)

• 33pF looks like 3300 pF to the input signal• Prevents oscillations caused by high frequency noise

• Troubleshooting• Inject small AC signal on the left (input) side

• Trace signal through amplifier chain• Amplitudes should increase as you move to the right except

for X105 – no amplitude gain.

Left Channel of Tape Recorder (Part 2)

Complete Amp System• Complete channel of old tape recorder

• Troubleshooting• As with all circuits - If output has problems

• Check supply voltage, if OK• Check convenient Mid-point of circuit, if OK• Check a convenient midpoint of the remaining part of the circuit that

has the malfunction in it• Repeat until problem is found

• Frequency Response (aka Tone Control)• S2 used to select from two different R-C circuits for tone control (S1

selects Record or Playback modes)• Fig 5-23 shows coupling circuit between TP’s 21 and 28 in Normal Playback (below)

Complete Amp System• Complete channel of old tape recorder

• Frequency Response (aka Tone Control)• S2 used to select from two different R-C circuits for tone

control (S1 selects Record or Playback modes)• At 100Hz (figure 5-24b –next slide)

» C113 reactance approx. = 39k ohms» C114 reactance approx. = 390k ohms

• As the frequency increases the signal feeding the second two stage amplifier increases. The circuit acts as a High Pass filter. See below

• S2 in chrome position connects different RC coupling

Analyzing the Tone Control