ENGR325 Electronics II
Lab #6 – Frequency Response of Amplifiers
· 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.
1. Last week we designed an amplifier of the form shown in Figure 4.53 of the text. Our resistor values were R1=200kΩ, R2=100kΩ, RD1=1kΩ, RS1=240Ω, and RS2=620Ω. Our Wavetek source resistance Rsi=50Ω. We will use a load resistor RL = 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 CS 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 CS so that it will cause the lower cutoff frequency of 100Hz.
Choose CS so that 1/(2πf)CS =(1/gm2||RS + RL) 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 CC1 so that 1/(2πf)CC1 = Ri + Rsi where f = 10Hz
Choose CC2 so that 1/(2πf)CC2 = Ro + RL 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 (fL) and the -3dB amplitude. Also, record the phase difference between the input and output waveforms.
Compare the measured fL with the 100Hz that you expected from your selection of CS. Compare the measured phase difference with the 450 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 fH. In class we understand that fH 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 450.
4. Finally, connect a 0.05μF capacitor CL in parallel with RL. Calculate the frequency at which this capacitor will drop the output by 3dB. Then measure the new fH and compare your measured result with the expected value from your calculations. Is the phase difference 450 at this frequency?
5. Model your circuit with CL 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.