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15th January 2020, 01:09 #21
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Re: Finding the input impedance of CCCS
Pretty immediate in this case. The output resistance with feedback removed is R1, the loop gain with port (terminals of RL) shorted is A and the loop gain with port open is 0. The result is R1*(1+A).
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15th January 2020, 19:09 #22
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15th January 2020, 21:57 #23
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Re: Finding the input impedance of CCCS
With a current for its input then its input voltage barely changes. Then its input impedance at low frequencies is extremely low (a virtual ground).

16th January 2020, 03:53 #24
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Re: Finding the input impedance of CCCS
I don't understand your statement. Assuming an ideal op amp and no reactive components, the voltage is directly proportional to the input current according to my calculations above unless they are in error. The input impedance should be as given in my previous post unless I goofed in my calculation.
RatchHopelessly Pedantic

16th January 2020, 07:59 #25
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Re: Finding the input impedance of CCCS
The admittance matrix for the circuit can be written by inspection, then inverted. The (1,1) element of the inverse is Zin:
Last edited by The Electrician; 16th January 2020 at 07:57.

16th January 2020, 08:19 #26
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Re: Finding the input impedance of CCCS
Hi,
I agree with audioguru.
The voltage will be almost zero.
But yes, even if small it will be proportional to input current.
KlausPlease don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.

16th January 2020, 15:53 #27
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Re: Finding the input impedance of CCCS
Hopelessly Pedantic

16th January 2020, 17:04 #28
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Re: Finding the input impedance of CCCS
Sorry, I don't understand these last posts. It has been already proven that Zin = R2/(1+A*B) that means Zin decreases as A increases.

16th January 2020, 17:58 #29
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Re: Finding the input impedance of CCCS
Amplification does not increase. The opamp already has a gain of almost infinity for a modern opamp. But some old opamps do not have much gain. Aren't you talking about modern opamps?
The input signal is a very high impedance current source and it is feeding the extremely low input impedance of the opamp inverting input. Then the input signal voltage is so low that we say it is zero.

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16th January 2020, 19:35 #30
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Re: Finding the input impedance of CCCS
I don't know where the formula Zin = R2/(1+A*B) comes from, or what the definition of A and B are. However, both calculations of The Electrician and myself agree on what Zin is. Yes, Zin approaches zero as the op amp amplification approaches infinity. What don't you understand about my derivation of the nodal equations of the circuit?
Ratch
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Amplification changes as op amps age or are replaced. High values of gain of several thousand or million are not near infinity. I am not focused on any particular op amp. Both the plus and minus terminals of an op amp are very high as is any voltage amplifier. The circuit under discussion has a very low input impedance, however.
RatchHopelessly Pedantic

16th January 2020, 21:47 #31
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17th January 2020, 03:20 #32
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Re: Finding the input impedance of CCCS
I had a hard time following your calculations, but it looks like you came up with the same solution as The Electrician and I did. I think the superposition method is on solid theoretical ground, but it entails multiple times as much work because you have to do multiple calculations for each excitation source.
RatchHopelessly Pedantic

17th January 2020, 06:02 #33
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Re: Finding the input impedance of CCCS
Looks to me the shortest solution was given in #5

17th January 2020, 17:26 #34
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17th January 2020, 18:14 #35
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17th January 2020, 19:22 #36
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17th January 2020, 20:36 #37
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17th January 2020, 20:50 #38
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Re: Finding the input impedance of CCCS
Yes, I read it. That's not a problem. I'm not saying other derivations are wrong, I said mine was correct and simple. Also, the result is arranged in a useful for design purposes way without need for algebraic manipulations.
Don't we all want to have young eyes?Last edited by sutapanaki; 17th January 2020 at 21:08.

17th January 2020, 21:29 #39
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Re: Finding the input impedance of CCCS
Yes, I did have trouble reading that light script and small scrawl. Sorry for the mistake.
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