Download Lab02_PartB - Weber State University

1
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. You don’t have to include detailed
analyses in your report for Part A. Please provide a table/summary showing calculated,
simulated and experimented values of overall gain GV, RE (RS) and RC (RD) of 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) An example of NPN common emitter and (b) An example of NMOS common
source amplifier circuit with coupling capacitors
First build and verify that the amplifier works as shown in Part A.
Alternatively, you can replace RE (RS) with a current mirror you built in Lab1. Next, work on the
frequency response of this amplifier as follows:
Weber State University
Naik
EE3120 Microelectronics II
Suketu
2
1.1 Calculation of pole frequencies
Calculate the pole frequencies are comprised of three lower band frequencies caused by each of
the discrete capacitors (DC blocking caps CC1 and CC2 and the bypass cap CE or Cs and the high
cutoff 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.
Alternatively, you can use eq. 9.83 and example 9.8 to obtain an accurate estimate of fH for both
MOSFET and BJT.
The parameters can be found on the datasheet or the model of the transistors. If there is
insufficient data, then you can omit the calculation of fH.
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 in the datasheet of the transistor.
L1: What are the approximate lower and higher cut-off frequencies of the amplifier (omit
the calculation of fH if insufficient data)? 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. Next, estimate fL and fH.
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 small range of frequencies 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!
Weber State University
Naik
EE3120 Microelectronics II
Suketu
3
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
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 calculated and simulated
values. 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 also 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 calculation and simulation?
(c) Perform the frequency sweep to locate both the upper and lower corner (cut-off) frequencies
of the amplifier gain (where the mid-band gain drops by 3 dB). The best way to do this is to find
the cut-off frequency at 0.707 x |Vout| at mid-band (say 10 kHz). Sweep the frequency to the
left of center to find fL and to the right of the center to find fH.
L5: What are the approximate measured cut-off frequencies, fL and fH of the amplifier?
L6: In your report, create a table with calculated, simulated and measured values for fL
and fH. Also, explain any discrepancies.
Weber State University
Naik
EE3120 Microelectronics II
Suketu