ECE 3235 Electronics II



EE 3235 Electronics II

|Experiment # 7 |

Feedback effects on Transient and

Frequency response

Note: Lab 8 is going to be continuation of the lab 7. You will re-using the circuit you build in lab 7. So, if you want to save effort, you can keep the built circuit in the lab until next week for lab 8.

1. Base Amplifier

The first part of the experiment is to construct and characterize an amplifier whose poles are set accurately by capacitors and resistors. First connect the circuit shown in Figure 1 using 741 OpAmps and ±15 V power supply voltages. Measure, and record the exact value of each component in the circuit. The OpAmps can be assumed to be ideal over the frequency range of this experiment. The purpose of the unity gain buffer amplifier is merely to make computations easier (why ?).

[pic]

Figure 1. Base Amplifier

Measure the gain and phase characteristic of the amplifier by applying a sinusoidal input waveform. Note that the DC or low-frequency gain is approximately –100 (why?). Since the power supply voltages are ±15 volts, you must keep the input voltage well below 150 millivolts peak. Always observe the output voltage to insure that there is no distortion.

Measure the phase shift at each frequency with your oscilloscope. Note that as the gain drops with increasing frequency, so you can increase the input level, but always check for distortion when you do so. Take enough measurements to enable you to later draw accurate gain and phase plots. You should take particular care to measure enough values at and near the pole frequencies, which is where the response will change most rapidly with frequency. These pole frequencies are in the vicinity of 1, 3, and 4 Kilohertz. A suggested sequence of frequencies is every hundred Hertz from 100 Hz to 1 KHz and every kilohertz from 1 KHz to 10 KHz.

2. Closed Loop Uncompensated Base Amplifier

Connect a voltage feedback network around the base amplifier, as shown in Figure 2

[pic]

Figure 2. Uncompensated Parallel voltage Feedback Amplifier (note that this is a parallel voltage feedback amplifier, therefore ideally this should be modeled by a Trans-resistance amplifier defined by voltage over current, however you can transform it to a voltage amplifier defined by voltage over voltage, in which case the feedback coefficient will change accordingly)

The triangular block represents the base amplifier of Figure 1. Terminals x and y should be open circuited. Note that the 200 K-ohms resistor labeled Ro in Figure 1 should not be removed.

First, let Ra = 1K-Ohm and Rb = 3.3K-Ohm (measure their exact values). Observe the output with your oscilloscope with Vin = 0 (short circuit the input). Does the feedback amplifier oscillate? If so, sketch the waveform at Vout and record peak amplitudes (positive and negative, with your scope on dc) and period. Now change Rb to 10K-Ohm (measure its exact value). Measure the low frequency gain with a sinusoidal signal applied to the input. Then you do frequency sweep to determine if the gain peaks. Measure the peak gain and the frequency at which it occurs.

Change the sinusoidal input to a 100 mV 50Hz square wave and observe the output with your oscilloscope. Sketch Vin (t) and Vout (t), being careful to note any “ringing” around the transitions of the waveform. You might have to increase the sweep speed and/or the beam intensity of your oscilloscope to properly see the ringing. It might also prove convenient to use delayed sweep. Feel free to change the frequency of the input square wave (but don’t cut off any of the ringing). Record the steady state output voltage (flat portion of the waveform), the first and second (+ and -) peaks, and the ringing period. Also, note the input level(s).

3. Cadence simulation

Build the circuit shown in Figure 1 and 2 in Cadence and simulate their frequency and transient response. Compare your simulation results with your measurement results. Also, for transient analysis, as discussed in class, it is preferred to give a pulse with a time-duration of 50 to 100 times of 0.35/B, where B denotes the bandwidth of the amplifier. What is the bandwidth B for the amplifier in Figure 1? (B should be the lowest break frequency or pole frequency you derived and measured before, why?).

Cadence tips

1) Use VSIN from AnalogLib for AC/frequency analysis (note that AC amplitude is set to 1 V)

2) Use VSOURCE from AanlogLib for transient analysis (select PULSE for type and set the pulse properly).

3) Use dB20( ) calculator statement if you want to convert the gain into dB format (to do that click Output in the analog design window, select setup, give a name for gain, type in dB20(VF(“/Vout”) supposing you denoted your output voltage by Vout, and finally click ok). Note the simulation results will give you the gain in terms of dB instead of the numerical value.

4) Similarly, use phase( ) calculator statement if you want to find phase of the output, note that you need to subtract 180 to get the actual phase.

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