Find voltage gain and input resistance for op-amp

In summary, the input resistances for the given ideal op amp circuits are both 10kOhms. The voltage gain for the first circuit is -10, and for the second circuit it is -10k. The ground-connected resistors can be ignored when calculating the input resistances and voltage gains due to the virtual ground property of high-gain op amps with negative feedback.
  • #1
ongxom
26
0

Homework Statement


Find the input resistances and voltage gains for those ideal op amps
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Homework Equations





The Attempt at a Solution


The original inverting circuit look like this :
jt260xU.png

we already have the equations :
input resistance = 10k
voltage gain = -r2/r1 = -10
For the first circuit :
it still a inverting op amps, does the red marked 10k resistor get involved with input resistances ? I think it's not because it connected to the ground (virtual ?). R2 is 100k so the equation for voltage gain remains the same as the original circuit .
Second circuit:
there is no current in red marked 10k resistor, input resistance is unchanged (10k), voltage gain remains (-10k)

I find it is difficult to calculate using op amps characteristics, can I use voltage node method to find the voltage gain, which node should I choose. Are those ground connected resistor have no effect on the circuit input resistances and voltage gain at all ?
 
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  • #2
You are correct that because of the virtual ground property (especially with ideal opamps), the input resistances of these circuits is 10kOhms. You have a typo in the gains, however. Can you see what your typo is?
 
  • #3
berkeman said:
You are correct that because of the virtual ground property (especially with ideal opamps), the input resistances of these circuits is 10kOhms. You have a typo in the gains, however. Can you see what your typo is?

Yes, for the second circuit the gain is -10.
 
  • #4
So can we say in those cases : In an inverting op-amp, any resistors connected to the ground can be ignored when calculate ?
 
  • #5
ongxom said:
So can we say in those cases : In an inverting op-amp, any resistors connected to the ground can be ignored when calculate ?

I wouldn't say that -- it's a bit too simplified and not always true. Instead, understand what the "virtual ground" means. Can you tell us what is going on with the "virtual ground" property of a high-gain opamp with negative feedback means?
 

1. What is an op-amp?

An op-amp, short for operational amplifier, is an electronic component that amplifies the voltage difference between its two input terminals. It is commonly used in many electronic circuits for its high gain and versatile functionality.

2. How do you calculate voltage gain for an op-amp?

The voltage gain for an op-amp is calculated by dividing the output voltage by the input voltage. It is typically represented by the symbol "A" and is measured in decibels (dB). The formula for voltage gain is A = Vout / Vin.

3. What is the significance of voltage gain in an op-amp?

Voltage gain determines how much the output voltage of an op-amp will change in response to a change in the input voltage. A higher voltage gain means a smaller input voltage can produce a larger output voltage, making the op-amp more sensitive and versatile in amplifying signals.

4. How do you find the input resistance for an op-amp?

The input resistance for an op-amp is calculated by dividing the input voltage by the input current. It is typically represented by the symbol "Rin" and is measured in ohms (Ω). The formula for input resistance is Rin = Vin / Iin.

5. Why is input resistance important in an op-amp circuit?

Input resistance determines how much current an op-amp will draw from the source of the input signal. A higher input resistance means less current will be drawn, resulting in less signal degradation and a more accurate output signal. It also affects the overall performance and stability of the op-amp circuit.

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