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Lab 02: Part B
Amplifier Frequency Response (20 points)
NOTE:
1) Please refer to Part A if you have not already built an NPN-based common emitter
amplifier or NMOS-based common source amplifier. Note that VCC or VDD = 10 V (instead
of 15 V as done in Lab 6 in EE3110), IC=0.5 mA (instead of 1mA) and RL= 100 kΩ (instead
of 10 kΩ) which changes the original design. You don’t have to include detailed analyses in
your report. Please provide a table/summary that shows overall gain GV, and the values for
RC (RD) & RE (RS) for your amplifier.
2) The procedure for analyzing the frequency response is same for both amplifiers
(common-emitter or common-source).
Objectives
The purpose of this lab is analyze the frequency response of an NPN-based common emitter
amplifier (or NMOS-based common source amplifier).
(a)
(b)
Fig. 1 (a) NPN common emitter and (b) NMOS common source amplifier circuit with
coupling capacitors
First build and verify that the amplifier works as shown in Part A
(EE3110 Lab 5 and 6 http://faculty.weber.edu/snaik/EE3110_labs.htm).
Set V+=10 V. Alternatively, you can replace RE (RS) with a current mirror you built in Lab1.
Next, simulate the frequency response of this amplifier as follows:
Weber State University
EE3120 Microelectronics II
Suketu Naik
2
1.1 Calculation of pole frequencies
Pole frequencies are comprised of three lower band frequencies caused by each of the discrete
capacitors (DC blocking caps CB and CL and the bypass cap CE) and the higher frequency caused
by the internal capacitors of the device.
Lower corner (cut-off) frequency (fL)
If the three pole frequencies are spaced far apart, then you can approximate fL= fp2.
For MOSFETs, see eq. 9.4, for BJTs, see eq. 9.14.
Higher corner (cut-off) frequency (fH)
Please refer to eq. 9.54 and example 9.3 to calculate fH for MOSFET.
Please refer to eq. 9.58 and example 9.4 to calculate fH for BJT.
Note that the amplifier bandwidth can be calculated as BW=fH-fL. Another important
parameter is the gain-bandwidth product or the unity grain frequency (frequency at which
gain=1 or 0dB), fT. fT can be found on the datasheet of the transistor.
L1: What are the approximate lower and higher cut-off frequencies of the amplifier as
calculated above? What is the unity gain frequency, fT?
1.2 Simulation
Hopefully at this point you will have run the transient analysis and built the circuit to verify that
the amplifier shows mid-band gain (AM) = the target value.
1.2.1 Frequency Response
Now apply 1V AC source to the circuit and run an AC simulation to observe the frequency
response (transfer function). Start with smaller range and increase the range till you find the -3dB
corner (cut-off) frequencies. Increase the number of points in your simulation setup to get good
resolution. Use the DB() and P() functions to plot the gain in dB and the phase in degree.
Chances are that the software (LTSpice or Multisim) will do the conversions for you. Use
cursors; put one cursor around the mid-band gain, 20*log10(AM) and the other cursor on -3dB
points (e.g. if AM=44 dB, look for -3dB points around 41 dB on left and right). Note the
frequencies at these points.
L1: Plot the simulated gain and phase transfer functions as the bode plot, and show these in
your lab report. Find and report the low and high -3dB corner (cut-off) frequencies in Hz.
1.3 Experiment
Don’t forget to check the multi-meter mode before you measure a current or voltage, if it
is set incorrectly you will blow a fuse!
We will utilize frequencies in the MHz range. At these frequencies, the parasitic effects (both
capacitances and inductances) of the breadboard, wires, pins of the discrete transistors and other
components become significant as they can cause additional poles to appear in the frequency
Weber State University
EE3120 Microelectronics II
Suketu Naik
3
response. In order to minimize the parasitic effects, use the shortest possible wires and clip the
terminal wires of your components and make them as short as possible.
1.3.1 Initial Measurements
(a) Measure the DC voltages and currents at all nodes (base, collector and emitter) of the circuit
without connecting any AC source. Compare these measurements with calculations. You should
have a good match between the calculated, simulated and measured DC values.
L3: What are the DC voltages and currents at each node?
(b) Now, apply a small (less than 0.01V peak-to-peak) voltage to the input at 10 kHz. Observe
the amplifier output and, if necessary, reduce the magnitude of the input until the output shows
no distortion (you can use the Math function on the o-scope to take FFT of the output to make
sure that the odd harmonics are at very low level). You may need to use the 10x probe if the gain
is too high (be sure to multiply the measured values by 10 when plotting your data). Calculate
the gain of the amplifier at this peak to peak input voltage.
L4: What is the measured value of the overall gain, GV (between input and output)? How
does it compare with your calculations and simulation?
(c) To get an idea of the overall transfer function, perform a quick frequency sweep to locate
both the upper and lower corner (cut-off) frequencies of the amplifier gain (where the midband
gain drops by 3 dB).
L5: What are the approximate measured cut-off frequencies of the amplifier?
1.3.2 Amplifier Transfer Function
In order to compare the frequency response of the amplifier between the simulated and measured
values, we need to measure both the gain and the phase.
These measurements will be used to create the bode plot. Starting at a frequency two decades
below the low corner frequency (fL/10), measure the gain and the phase (if you are unsure about
how to measure the phase from time-domain data, please refer to
http://eeshop.unl.edu/pdf/OscilloscopeTutorial--PhaseMeasurement.pdf ). Repeat the same
measurements for the rest of the frequency range, up to about 10fH if possible. Take enough data
so that they can be plotted and compared with simulated data.
Take as many as points as possible and spread them out across the entire band. Be sure to get a
lot of points near the corner frequencies.
L6: Plot the measured gain and phase transfer functions as the bode plot, and include them
in your lab report (recall that amplitude response is plotted as 20*log(gain) vs log(freq) and
phase response is plotted as degrees vs log(freq). Plot the simulated and measured values
together. Did you get a good match?
Weber State University
EE3120 Microelectronics II
Suketu Naik
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Now replace the probe with a short coax cable (Note: standard coax cable has a capacitance of
about 30 pF/ft and resistance of 50 Ω.) Do a quick frequency sweep (you don’t need to show a
bode plot for the coax cable). Do you notice any difference in the corner frequencies? What was
the difference and what do you think caused it?
L7: How does the frequency response of the amplifier change with the standard coax cable
at the output?
In addition to the bode plots, create a table with calculated, simulated and measured values
of the pole frequencies in your report. Comment on the behavior of the amplifier when
coax cable is connected at the output.
1.3.3 (Optional) Input and Output Resistances of the Amplifier
In order to use the amplifier in a given system (remember that the amplifier will be connected to
filters, switches, mixers and/or A/D or D/A converters in a signal processing chain), we need to
characterize its input and output resistances. That way, we can connect the amplifier with
expected signal degradation (or improvement) due to impedance mismatch.
In order to measure the input and output resistances of the amplifier, we can use a test resistor.
The idea is to measure the potential drop across it, calculate the current from the measured
values and then take the ratio of input (output) voltage and the current to calculate the input
(output) resistances.
(1) To measure the input impedance of the amplifier, use Vsig = 1 Vamp at 10 kHz (or any
mid-band frequency) if there is no distortion. Otherwise set the input voltage to a value
that produces no distortion.
(2) Connect 1 kΩ resistor between the function generator and the input of the amplifier
(right before the input DC blocking cap). Carefully measure and record the voltages at
the two nodes of the resistor (O-scope measurements are more accurate than the
multimeter however note that you will probably blow a fuse if you use a single probe and
measure the potential drop across the resistor on the o-scope. Instead, use two probes to
measure the voltages. Alternatively, use the multimeter to find RMS voltage drop).
Measure and record the values as V1 and V2 as accurately as possible. Also be sure to
record the exact value of the resistor and call it R. From the measurements, calculate the
value of Rin as,
Rin = (Vsig / I) - R; where, I=[(V1-V2)/R)]
Similarly, measure the value of output resistance Ro. For output resistance, you should short the
input of the amplifier to ground (right before the input DC blocking cap), and connect Vsig = 1
Vamp at 10 kHz at the output terminal (right after the output DC blocking cap).
L8: What are the measured input (Rin) and output resistances (Ro) of the amplifier?
Include the measured values of the input and output resistances in your report.
Weber State University
EE3120 Microelectronics II
Suketu Naik