Closed Loop Op amps Flashcards
What is one main thing an open loop op amp can be used for? explain
-a switch
-the behaviour of open loop op amps are unpredictable when saturated.
What is the purpose of feedback?
By feeding back to the inverting input, the voltage differential between the inputs is decreased.
Briefly explain the virtual ground concept
because Vi is approx 0V (which is almost ground voltage)
-it is not physically connected to ground
-feedback forces the inputs to be at the same potential
-the virtual ground concept is useful where negative feedback is involved, making analysis easier
What is the gain of a non-inverting op amp?
Vout/Vin = 1 + Rf/Ri
What is the gain of an inverting op amp?
Vout/Vin = - Rf/Ri
In the inverting amp, where is the non-inverting input connected to?
Ground
Op amps have large voltage gains explain why
High input impedance, low output impedance (almost 0)
Ideal op amps have infinte voltage gain
What is the equation for dB?
20log10(Av)
Define a voltage follower
An op amp with a gain of 1
Vo = V1
Why is the virtual earth a concept and not applied to the op amp?
The short would make the voltage gain very small.
Why is input current 0?
Due to high input impedance, no current flows in the inputs
-all current is directed through the feedback resistor
Give the differences between the virtual ground and real ground
VG:
-voltage is approx 0
-not electrically connected to ground
-made for easy calculation purposes
RG:
-Voltage IS 0
-Physically connected to circuit
-acts as reference point for entire circuit
Where does the virtual ground occur on an op amp
At the node where Rf and Ri connect on the inverting input
Define loop gain
The fractional change in closed loop gain per unit change in open loop gain
-to derive this we differentiate the closed loop transfer function in terms of open loop gain
Give the formula for loop gain
(1+ßAv) but for feedback to be effective,
(1+ßAv) = ßAv
