PROBLEMS
1)
Design a Class A emitter follower for
maximum power to an 8Ω load, as shown in the
figure. Use 2N3904 transistors (maximum collector
dissipation is 0.5 watts, maximum collector current
is 200ma).
Answer For Class A operation the collector dissipation capability should be more than twice the load
power. The specified device limitation of 0.5 watt means the load dissipation should be less than 0.25
watt. For an 8Ω load dissipating 0.25 watt (Ave.) the peak load current then would be 250ma. The
maximum collector current of the specified device
however is 200ma; this is the trial design
specification that will be used.
*Prob #1
VS 4 0
SIN( 0 1.5 1K)
For a nominal 200ma Q1 current select
RB0 1
27
RB=(6-0.7)/0.2 ≈ 27Ω (1 watt); neglecting Early
Q1 1 1
2
Q2N3904
Effect this will be the nominal Q2 collector current.
Q2 3 1
2
Q2N3904
Q3 5 4
3
Q2N3904
A PSpice netlist is to the right, and DC voltages and
CO3 6
100U
currents follow below.
RL 6 0
8
V+ 5 0
DC
6
NODE VOLTAGE
V- 2 0
DC
-6
(1) -5.1392 (2) -6.0000 (3) -.8579
.TRAN 0.1M 2M
(4) 0.0000
(5) 6.0000
(6) 0.0000
.LIB EVAL.LIB
.PROBE
NAME
Q1
Q2
Q3
.OP
IC
1.85E-01
1.96E-01
1.93E-01
.END
Q2 and Q3 currents, together with load current, are plotted below.
Introductory Electronics Notes
The University of Michigan-Dearborn
90-1
Copyright © M H Miller: 2000
revised
2)
Estimate the performance of the Class B
amplifier circuit drawn to the right. Compare your
estimates against the results of a PSpice computation.
Transistors are 2N3904 (NPN) and 2N3906 (PNP)
respectively. Suggestion: Use a nominal signal
frequency of 1 kHz and amplitude 4 volts.
Answer Estimate the base voltage of Q3 ≈ 0.7v; current in the 1.8KΩ bias resistor then is 0.7/1.8 =
0.39ma. This also is the estimate for quiescent Q4 emitter current. The Q4 emitter voltage is 0.7
+(0.7/1.8) 12 = 5.37v. The collector current of Q1 is estimated to be (12-0.7)/5.6 = 2.02ma; this also
is the estimate for the Q2 collector current (ignoring the Early Effect). Approximating the inverting
amplifier as idealized the estimate for the voltage gain is -12/10 = -1.2.
A PSpice netlist for the circuit follows, after which the computed operating parameters are listed for
comparison.
*Problem 2
VAC
8
RAC
8
CIN
1
RB1
2
RB2
4
Q1
7
Q2
3
RI
7
NAME
MODEL
IC
0
1
2
0
2
7
7
0
Q3
3
2
0
Q4
6
3
4
Q5
0
3
4
CL
4
5
RL
5
0
VCC
6
0
DC
.TRAN .01M 3M 0 .01M
.LIB EVAL.LIB
.OP
.PROBE
.END
SIN(0 4 1K 0 0 0)
10K
100U
1.8K
12K
6
Q2N3906
6
Q2N3906
5.6K
NODE VOLTAGE
(1) 0.0000
(2) .6886
(5) 0.0000
(6) 12.0000
(3) 6.1126
(7) 11.2760
(4) 5.4731
(8) 0.0000
Q1
Q2N3906
-1.99E-03
Q3
Q2N3904
2.54E-03
Q4
Q2N3904
3.96E-04
Q2
Q2N3906
-2.54E-03
Q2N3904
Q2N3904
Q2N3906
100U
50
12
Q5
Q2N3906
-8.49E-12
The transient response to a sinusoidal signal, 4SIN(2000πT), is plotted next
Introductory Electronics Notes
The University of Michigan-Dearborn
90-2
Copyright © M H Miller: 2000
revised
3)
Estimate the performance of the Class B
amplifier circuit drawn to the right. Compare your
estimates against the results of a PSpice computation.
Transistors are 2N3904 (NPN) and 2N3906 (PNP)
respectively. Suggestion: Use a nominal signal
frequency of 1 kHz and amplitude 4 volts.
Answer
Introductory Electronics Notes
The University of Michigan-Dearborn
90-3
Copyright © M H Miller: 2000
revised
Approximate the inverting amplifier as an idealized device and estimate the voltage gain as (roughly)
3.3/1. Estimate the maximum symmetrical output voltage swing as ≈ 5v.
A PSpice netlist and computations follow. Note that there is no correction for crossover distortion in this
circuit/
*Problem 3
VAC
RAC
CIN
RB1
RB2
Q1
RC
RE
Q2
1
1
2
3
8
5
8
4
8
0
2
3
0
3
3
5
0
5
SIN(0 1 1K 0 0 0)
1K
100U
10K
68K
4
Q2N3904
3.3K
470
6
Q2N3904
NODE VOLTAGE
(1) 0.0000
(2) 0.0000
1.4439
(4) .7672
(5) 6.6494
(6) 6.5007
0.0000
(8) 12.0000
NAME
Q1
Introductory Electronics Notes
The University of Michigan-Dearborn
Q3
0
5
6
CL
6
7
RL
7
0
VCC 8
0
DC
.TRAN .01M 3M 0 .01M
.LIB EVAL.LIB
.OP
.PROBE
.END
MODEL
(3)
IC
11
(7)
Q2
90-4
Q2N3906
100U
50
12
Q2N3904
Q2N3904
Q2N3906
1.62E-03
-1.01E-11
1.54E-
Q3
Copyright © M H Miller: 2000
revised
4)
Estimate the performance of the Class B
amplifier circuit drawn to the right. Compare your
estimates against the results of a PSPICE
computation. Transistors are 2N3904 (NPN) and
2N3906 (PNP) respectively. Approximate the circuit
as an inverting amplifier with an 8.2 KΩ shunt-shunt
feedback to estimate the voltage gain to be about 8.
Suggestion: Use a nominal signal frequency of 1
KHz and amplitude 1volts. How reliable is this
circuit, e.g., as a function of temperature.
Answer
DC Estimates:
The nominal voltage at the base of Q1is estimated as 1.4v. Actually this
probably will be on the high side because as a Darlington driver Q1 will have a small collector current and
so a smaller junction voltage. Nevertheless we use the 1.4 v estimate. The current in the 820Ω will be
about 1.4/.82 = 1.7 ma, and this is likely to be quite a bit larger than the Q1 base current. With this
assumption the load voltage (470Ω) is estimated to be -12+1.4+14 = 3.4v, with a corresponding DC load
current of 7.2ma. The Q3 emitter current then would be ≈ 7.2+1.7 = 8.9ma. The Q2 collector current is
(12-3.4-0.7)/8.2 = 0.96ma
A PSpice analysis (see netlist below) computes a Q1 base voltage of -10.8 v, corresponding to a junction
voltage drop (Q1 and Q2) of 1.2v compared to the estimate 1.4v. The load voltage computed is 1.19v
compared to the estimated 3.4v. The computed Q2 collector current is 1.2ma; 0.96 ma estimated. The Q3
emitter current computed is 3.96 ma compared to the estimate of 8.9ma.
The inverting amplifier may be regarded as having
shunt feedback of 8.2KΩ, with a 1KΩ input resistor
providing an estimated gain of -8.2.
*Problem 4
VAC 1
RAC 1
CIN 2
0
2
3
SIN(0 {MAG} 1K 0 0 0)
1K
100U
Q1
RB1
RB2
Q2
RC2
Q3
Q4
3
8
3
4
5
5
5
4
7
3
6
5
7
7
8
8
6
6
RL 6
470
.PARAM MAG = 1
VCC
7
0
DC
12
VEE
8
0
DC
-12
.TRAN .01M 5M 0 .01M
.LIB EVAL.LIB
.STEP PARAM MAG LIST 1 1.2 1.4
.OP
.PROBE
.END
Q2N3904
820
8.2K
Q2N3904
8.2K
Q2N3904
Q2N3906
Introductory Electronics Notes
The University of Michigan-Dearborn
0
90-5
Copyright © M H Miller: 2000
revised