Calculations/Figures
Feedback Ratio for Voltage Series Configuration
Gain Desensitization
Bandwidth Extension
Results
The results of the experiment in step 1 and step 2 are below in Table 2 and Table 3 respectively. The gain of the negative feedback circuit was designed to be 100 V/V. The high frequency 3db point was 940Khz. When feedback was removed from the circuit the gain increased to 1509 V/V and the high frequency 3db point decreased to 185 KHz. The applications of this are discussed in the individual discussion section.
Table 2: Feedback circuit values (step 1)
Table 3: Non-Feedback circuit values (step 2)
Grant’s Discussion
This time around we got to hook up a circuit on the breadboard which was actually fairly complicated and compact. This circuit needed to cover a very small area because if even a small amount of outside noise got into the circuit it would severely distort our measurements. This was the case because we were inputting such a small signal in the order of 50 mV since we were expecting a large voltage gain on the output.
The first step in this lab was to connect the amplifier which we did and then adjust the feedback resistance to find where the voltage gain Vo/Vs went to 100. We found this resistance to be 16.32 kΩ; this gave us an output voltage of 10.0 V and our input voltage was 100 mV, as can be seen in figure 1. This is a voltage increase of 100 times. This was consistent with the lab write-up. Next we found the fH where the signal was at its 3db point. This value was 940 KHz which again was close to what was expected and therefore our experimental value was correct. Then we predicted Acircuit gain from the equation given in the lab report. The value we obtained was 100 and can be seen in the calculations above.
Next we changed out value of RL from infinity to the value of RFEEDBACK2 which was determined in the last part of the lab. Hooking up a 16.32 kΩ resistor to the circuit in place of RL and RFEEDBACK2 as infinity gave the circuit to do this next part. We then hooked up the oscilloscope to read the input value and output value. With an input of .01 V we read the output at 15.09 V as seen in figure 3 above. This, when calculated, gave a voltage gain of 1509 V/V. We were expecting a value well above 1000 V/V and obtained one so we were done with this section of the lab. We again needed to find fH for this circuit where the circuit was at it 3db point, this was at 185 KHz. The reason the voltage gain was so big was because we had eliminated the feedback in the circuit.
Some applications of this kind of circuit are ideal for a wide range of applications including video, communication, and imaging. When only a small signal can be provided and a large signal is needed to drive another electrical component this kind of circuit could be used and designed to supply the right voltage over a large range of devices.
Matt’s Discussion
In this lab the effects of negative feedback on a circuit are clearly shown. The feedback decreases the overall gain of the circuit but allows for a much larger bandwidth. The common uses of feedback amplifiers are
Emily’s Discussion
In this lab, we demonstrated the effects of negative feedback on an amplifier. Analytically, we know that a feeding some signal back to the input of the amplifier stabilizes the gain. It will decrease the gain, while increasing the high 3dB frequency and the low 3dB by the same factor, which is dependant on the feedback by the above relations.
When we hooked up our circuit and finally got it working, we determined the RFEEDBACK value by increasing a variable resistor until we obtained the desired gain of 100V/V. We arrived at a resistor value of 16.32kΩ. The high 3dB frequency we obtained was 940kHz. The feedback factor we determined to be was .01.
For the second part, we obtained a voltage gain 1509V/V. This is consistent with the larger gain, by a factor of (1+AH). We determined the high 3dB frequency to be 185kHz, which is also consistent with the feedback increasing the high 3dB frequency by that same factor.
Negative feedback amplifiers are extremely useful in applications such as inputs to analysis software from thermocouples and RTD’s, They are used because they have a very stable, reliable gain, as well as a low output resistance.
Conclusion
In this lab, we demonstrated the difference between feedback and non-feedback amplifiers. The effect of the feedback circuit was to increase the bandwidth of operation of the amplifier. The trade off is in overall gain.