Electronics 3b - Op amp circuits

Question 1

What is the benefit of a Negative feedback circuit for an op-amp?

Answer 1

Circuits with negative feedback produce an output that varies linearly with changes to its input over the full operating range.

Essentially: output isn’t just saturated or off, the output varies according to the input respectively, no longer square output but the same or inverted version of the input with controllable gain.

Question 2

What is the issue with Positive feedback for op-amps?

Answer 2

We don’t really consider them as they are highly unstable.

Question 3

How does negative feedback work to produce a varying output that is linear to the input?

Answer 3

The voltage across the two terminals will do everything it can to produce the same voltage across both terminals.

Essentially, the input in this circuit is say 1V, then output of the circuit will also be 1V. The postive terminal input will be 1V multiplied by the gain of the op-amp (in this case without any resistor and because its a 741 op amp, gain is 10^5). This rise in gain, to keep the output the same (1V), then the negative terminal must be 1 / 10^5, so that the difference is almost nothing. Thus we have no difference and the output is almost the input.

positive terminal: 1V * 10^5
negative terminal: 1 / 10^5 = 10^-5
Vin = 1V
Vout = 0.999999V = 1V.

So Vin = Vout

Question 4

State the two ‘golden rules’ (assumptions) of op amps in the closed loop mode?

Answer 4

1. The output will do whatever is necessary to make the voltage difference between the two inputs zero.
2. No current flows into the two inputs (this isn’t strickly true for BJT transistors, but its almost negligable)

Question 5

State some examples of closed loop op-amps.

Answer 5

• Voltage follower
• Non-inverting amplifier
• Inverting amplifier
• Summing amplifier
• Differential amplifier
• Differentiator
• Integrator

Question 6

What is the general transfer function for a voltage follower, op-amp circuit?

Answer 6

Vin = Vout

Question 7

What is the general transfer function for a Non-inverting amplifier, op-amp circuit?

Answer 7

Vout = Vin (1 + (R1/R2)

R2 = Rf

Note:
* I1 = I2 as no current can go through the Op-amp. VF is also equal to Vin at the positive terminal so the difference is zero.
* Gain is affected by the reistors Gain, G or A = (1 + R1/R2)

Question 8

What is the general transfer function for a Inverting amplifier, op-amp circuit?

Answer 8

Vout = - (Rf/R1) Vin

Note:
* Current IF and I1 are the same as no current can enter the op-amp
* Notice that the positive terminal is connected to ground so Vf must be a virtual earth to get 0v across it.

Question 9

What is the general transfer function for a Summing amplifier, op-amp circuit?

Answer 9

Vout = - (Rf/R1 V1 + Rf/R2 V2 + Rf/R3 V3)

Question 10

What is the general transfer function for a Differential amplifier, op-amp circuit?

Answer 10

Vout = R2/R1 (V2 - V1)

notice how if R2 = R1, then gain is 1, and it only outputs the difference of the two voltages exactly as they where.

Question 11

What is the general transfer function for a Differentiator, op-amp circuit?

Answer 11

Vout = -RC (dVin/dt)

Note:
* how its derived using the capacitors function for I1 and being equal to the current I2 across the resitors equal to its voltage across there, relating to Vout.
* Also notice the virtual earth
* Swap the capacitor and resistor and you get the integrator circuit

Question 12

What is the general transfer function for a Integrator, op-amp circuit?

Answer 12

Vout = - 1/RC ∫ Vin dt

Note:
* How its derived from the capacitors function and resistors, related to the currents across them, since current can’t enter the op-amp.
* Also the virtual earth at VF
* Swap the capacitor and resistor and you get the defferntiator circuit