# Op Amp

Discussion in 'Homework Help' started by JasonMcG, Jun 7, 2014.

1. ### JasonMcG Thread Starter New Member

Apr 27, 2013
24
0
Hi

I need help with this problem and how to go about doing it.

A non-inverting amplifier based on the op amp LM 307 has a gain of 15 and RF = 140 kΩ. The operational amplifier specifications are: Ri = 2.0 MΩ, , Ro = 50 Ω, A(OL) = 160 V/mV, V(io) = 7.5 mV , I(B) = 250 nA, I(io) = 70 nA.
i) Determine the feedback factor of the non-inverting amplifier and hence or otherwise determine the closed loop voltage gain of the amplifier if the A(OL) is considered to be finite. [4]
ii) Calculate the worst case output offset dc voltage of the uncompensated non-inverting amplifier . [5]
iii) Determine the value of the offset minimizing resistor R(OM). [2]

Jason

2. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,956
1,131
We don't do homework for you, but we can (hopefully!) offer advice.
What thoughts have you had so far?

3. ### JasonMcG Thread Starter New Member

Apr 27, 2013
24
0
I'm not looking for answers but ways to go about solving these problems. Wanting to learn and understand. Where exactly does one start with this problem? I thought Rf is the feedback factor.

4. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,499
511
To me Rf is what I would label feedback resistor, the fact that it is 140 kOhm seem to support what I think. Is feedback factor measured in Ohms?

This seems relevant to feedback factor: http://www.ecircuitcenter.com/Circuits/op_bandwidth1/op_bandwidth1.htm

Last edited: Jun 7, 2014
5. ### JasonMcG Thread Starter New Member

Apr 27, 2013
24
0
The feedback factor is beta correct? Using the formula Gain=1+Rf/R1 I calculated R1 to be 10k and the feedback factor I worked out to be 0.066

6. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,956
1,131
Yes.
Your calculations would be correct for an ideal opamp having infinite gain and infinite input impedance. The results are a very good approximation to reality. However, to answer question 1 correctly you need to take into account both the actual input impedance and the finite gain as per spec. Draw out the circuit, including the input impedance, to see how that affects things.

Edit:
I'm unclear why you're told the amp 'has a gain of 15', and then in (i) you are asked to 'determine the closed loop voltage gain' ?

Last edited: Jun 7, 2014