# Voltage output of a Cascade Amplifier

Discussion in 'Homework Help' started by Jess_88, Apr 30, 2011.

1. ### Jess_88 Thread Starter Member

Apr 29, 2011
174
1
Hi guys I was wondering if I could get some help getting started with this question.

Given Rf = ∞, determine Vo. Given Rf = ∞, I am thinking that there is no current flowing through the resister and therefore no voltage. So would I neglect it from my calculations???

I'm also not sure how the input to the second out amp effects the circuit.... I'v only ever seen it connected to a negative input.

I have started my analysis, finding the current through the 5kΩ and The 15kΩ resistor.

I1 = 10mV/20kΩ

Again... I'm not sure if I'm my method if correct at all... due to Rf.

Thanks so much guys

Last edited: Apr 30, 2011
2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
Yes - since Rf=∞ you simply neglect it in the calculations.

You have two stages - the first is an inverting amplifier, the second a non-inverting amplifier. The overall gain magnitude is the product of the two gains - keeping in mind the inverting mode of the first stage. Will the overall gain have a positive or negative sign?

You don't need to go from first principles to find the gain. See sections 2.2 & 2.3 in the link.

http://en.wikipedia.org/wiki/Operational_amplifier_applications

Last edited: Apr 30, 2011
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3. ### Jess_88 Thread Starter Member

Apr 29, 2011
174
1
ah ok.

so for the first inverting amp
v1 = -10mV(15/5) = -30mV

and for the non-inverting amp
v2 = -30(1 + 6/2)

do I have to take into account the resistance at the output?
or is v2 = vo?

thanks so much for the quick reply 4. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
The amplifiers are probably meant to be considered as ideal & would notionally have zero output resistance. So the load resistance would have no impact on the overall gain - which you seem to have correctly resolved.

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5. ### Jess_88 Thread Starter Member

Apr 29, 2011
174
1
thanks so much for the help 