ENGR325 Electronics II

**Lab #6 – Frequency Response of Amplifiers**

**Objectives:**

· To experimentally determine the frequency response of an amplifier.

· To compare our experimental response with that expected from theory.

· To gain experience with creating Bode diagrams to represent the frequency response.

**Procedures:**

1. Last week we
designed an amplifier of the form shown in Figure 4.53 of the text. Our resistor values were R_{1}=200kΩ,
R_{2}=100kΩ, R_{D1}=1kΩ, R_{S1}=240Ω,
and R_{S2}=620Ω. Our Wavetek source resistance Rsi=50Ω. We
will use a load resistor R_{L} = 1kΩ. We used the BS170 MOSFETs.

This week we will look again at our coupling and bypass
capacitor calculations. I am suggesting
that we aim for a lower cutoff frequency of 100Hz. Since C_{S }is the largest of the
capacitors and potentially the most expensive, we will try to keep its value to
a minimum. Thus, we should choose C_{S}
so that it will cause the lower cutoff frequency of 100Hz.

Choose C_{S} so that 1/(2πf)C_{S} =(1/g_{m2}||R_{S }+
R_{L}) where f = 100Hz

Then we will choose our coupling capacitors so that they will have cutoff frequencies of about 10Hz and will not affect the gain at 100Hz.

Choose C_{C1} so that 1/(2πf)C_{C1} = R_{i}
+ R_{si} where f = 10Hz

Choose C_{C2} so that 1/(2πf)C_{C2} = R_{o} + R_{L} where
f = 10Hz

2. Build the
circuit. Begin by using a 10kHz sine wave input at an amplitude that is not causing
output distortion. Record the output amplitude.
Record the phase difference between the input and output waveforms. Then keeping the input amplitude constant,
lower the frequency until the output amplitude drops by 3dB or to 0.707 times
the recorded value at 10kHz. Record the frequency (f_{L})
and the -3dB amplitude. Also, record the
phase difference between the input and output waveforms.

Compare the measured f_{L}
with the 100Hz that you expected from your selection of C_{S}. Compare the measured phase difference with
the 45^{0} value expected at the -3dB down frequency.

3. Now calculate the expected high-frequency 3-dB down value due to the internal capacitances of the BS170 transistor using the capacitance values from the data sheet.

After making this
calculation we will experimentally look for the high frequency corner or cutoff
frequency f_{H}. In class we understand that f_{H} is caused by the internal MOSFET capacitances
and any load capacitance that we might have.
Check again the output amplitude for a 10kHz
input and record this amplitude. Record
the phase difference. Then increase the
frequency of the input looking for a frequency that will cause the output to
drop again be 3dB or to 0.707*recorded value.
The Wavetek will go up to 4MHz. The HP Function Generator will go up to
15MHz. We have other signal generators
that can go up to 100MHz if we find we need them. When you find the upper 3dB corner frequency,
record the frequency and the output amplitude.
Also, record the phase difference.
It should again be 45^{0}.

4. Finally, connect a
0.05μF capacitor C_{L} in parallel with R_{L}. Calculate the frequency at which this
capacitor will drop the output by 3dB.
Then measure the new f_{H} and compare
your measured result with the expected value from your calculations. Is the phase difference 45^{0} at
this frequency?

5. Model your circuit
with C_{L} included using Multisim and the
Bode plotter to obtain a Bode plot of amplitude and phase from 1Hz to
100MHz. On these graphs indicate where
you measured the -3dB frequencies in parts 2, 3, and 4. Comment on the agreement you see with the
simulation. Summarize the experience you
gained by exploring the frequency response of this amplifier.