Many circuits, particularly amplifiers, use negative feedback (nfb) in order to function reliably. The nfb changes the performance of the circuit to which it is applied, in most cases to the advantage of the user. It is important to understand these effects and establish their validity.
It is also important to realize that as the phase of the output changes, the feedback which is initially negative, must eventually become positive. When this happens, unless certain design margins are met, the circuit will become unstable.
Use the reference texts to deduce an expression for the voltage gain of an amplifier. This may be done for a voltage series, current series, voltage shunt or current shunt circuit.
Complete the questions below:
1. A non-inverting amplifier has an open loop gain of 120 000, an input impedance of 10 kΩ, an output impedance of 100 Ω and the desired closed loop gain is 100.
The signal source provides a 20 mV signal through an output impedance of 500Ω. The load resistance is 100Ω
Determine:
a) the approximate feedback fraction
b) The modified closed loop gain.
c) The output voltage.
d) The modified input impedance
e) The modified output impedance
f) The voltage across the load
2. If the open loop gain of an amplifier is 100 ±12 dB and the desired closed loop gain is 60 dB determine:
a) the approximate feedback fraction
Then use this value to determine
b) The maximum and minimum arithmetic gains
c) The maximum and minimum gains expressed as dB
Note Av dB = 20 log(arithmetic Av)
3. A transistor amplifier has an open loop gain of 500 and low and high corner frequencies of 1 kHz and 15 kHz respectively. If the closed loop gain is 25 determine the new corner frequencies and the % increase in bandwidth.