Lab 6: Emitter Follower Amps; Input and Output Impedance; Current
Mirrors
Group Member Names:
Pre-Lab Reading: PEFI sections: Figure 4.55, Figure 4.56, Figure 4.66, Figure 4.67.
Experiments:
1. Dual-Supply Emitter Follower Amp: Construct the circuit in Figure L6-1.
(a) Set up a 6Vpp 3kHz sine wave on the ELVIS Function Generator. Apply this signal to Vs.
Measure the peak-to-peak voltage at Vo with one scope channel and measure the peak-to-peak voltage at Vin (NOT
at Vs) with the other channel. Record your two measurements here along with the voltage gain Av = Vo/Vin. (Is Av
close to +1?)
Take a scope screenshot with both Vin and Vo displayed. After lab, print out this screenshot and staple it behind
the lab worksheet.
(b) Analytic problem: Use the lecture notes on emitter follower amps to estimate the input impedance of the amp.
Assume that β ≈ 179. (The datasheet for a 2N3904 lists 100 < β < 300.) Show your calculations here:
Rin = Beta + 1 / (Re || RL) Re = 3.3k, RL = 1k // (1 + 179)*(1/((1/3300) + (1/1000)) = 138kOhms
(c) Now measure the peak-to-peak voltage at Vs with one scope channel and measure the peak-to-peak voltage at Vin
with the other channel.
Record your two measurements here:
Use these two measurements to estimate the input impedance of the amp as viewed from node Vin looking
rightward. Use the “two-voltage” method as described in the notes from the first week of class. Here the source
resistor Rs functions as the “known” resistor. Show your calculations here:
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PIC2
(d) Analytic problem: Use the lecture notes on emitter follower amps to estimate the output impedance of the amp.
Assume that β ≈ 179. Show your calculations here:
Rout = Re || ((Rs / (Beta + 1))
(e) Measure the peak-to-peak voltage at Vo. This will be the “loaded output voltage”. Record your measurement
here:
Voi = 5V
Remove the 1kΩ load resistor. Measure the peak-to-peak voltage at V0. This will be the “unloaded output voltage”.
Record your measurement here:
Vof = 5.8V
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Use the loaded and unloaded output voltages to estimate the output impedance of the amp as seen at node Vo
looking leftward. Use the “two-voltage” method as described in the notes from the first week of class. Here the
load resistor RL functions as the “known” resistor. Show your calculations here:
Vloaded = Vunloaded*(RL/(Rout + RL)) = 174Ohms
2. Incorrectly-Biased Single-Supply Emitter Follower Amp: Construct the circuit in Figure L6-2. The idea is to
build an emitter follower amp that just uses a single +15V supply, but you will see that it does not work.
(a) Set up a 6Vpp 3kHz sine wave on the ELVIS Function Generator. Apply this signal to Vs.
Display the output Vo with one scope channel and and the emitter node VE with the other channel. Make sure
both channels are DC-coupled. Use the automated measurements section of the scope to display the Minimum
value of VE. Record the minimum value of VE here:
Vemin = 640mV
(b) This amp is supposed to be an emitter follower. Explain below why the emitter node was unable to “follow” Vin
over some portions of its waveform.
It wasn’t able to follow because it was tied to ground instead of -15V
3. Correctly-Biased Single-Supply Emitter Follower Amp: Construct the circuit in Figure L6-3.
(a) Set up a 6Vpp 3kHz sine wave on the ELVIS Function Generator. Apply this signal to Vs.
Display the input Vin with one scope channel the output Vo with the other channel.
Take a scope screenshot with both Vin and Vo displayed. After lab, print out this screenshot and staple it behind
the lab worksheet.
(b) This amp performs better than the last version. Explain below how the output is now able to “follow” Vin over ALL
portions of its waveform. Hint: Consider the relationship between nodes VB and VE and their respective DCmeans.
The base node is biased to positive DC, along with emitter node.
4. “Bad” Current Mirror: Construct the circuit in Figure L6-4. Note that by using discrete transistors, rather than
transistors that are all on the same IC substrate, we are violating the “well-matched transistors” assumption necessary
for proper operation.
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(a) Using the lecture notes, analytically estimate the value of the reference current Iref . Show your calculations here:
Iref = 0.96mA
(b) Measure Iload with your multimeter. (Remember this is the “bad mirror”. It may not match Iref .) Record your
measurement here:
2.7mA
(c) Warm up transistor Q1 by holding it between your thumb and forefinger. Describe what happens to Iload as you
do this:
ILoad decreases
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(d) Warm up transistor Q2 by holding it between your thumb and forefinger. Describe what happens to Iload as you
do this:
ILoad decreases again
5. “Good” Current Mirror: Borrow a CA3046 or LM3046 from the instructor. Construct the first version of the circuit
in Figure L6-5. Note that these transistors are well-matched because they are all on the same IC substrate.
(a) Measure Iload with your multimeter. Record your measurement here:
.12mA
(b) Modify the circuit into its second version by adding a 1kΩ resistor in series with the collector of Q2. Measure Iload
with your multimeter. Record your measurement here:
.02mA
(c) Modify the circuit into its third version by adding a 4.7kΩ resistor in series with the collector of Q2. Measure
Iload with your multimeter. Record your measurement here:
0.18mA
(d) If the collector of Q2 was acting like a voltage source, increasing the collector resistance by a factor of x10 (from
1kΩ to 10kΩ) should have decreased Iload by about a factor of 10. Conversely, if the collector of Q2 was acting like
a current source, the value of Iload should not have changed much. Do you think the collector of Q2 is acting like
a voltage source or a current source? Explain below:
The collector is acting like a current source
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