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Design >> Analog Design >> Passive circuit voltage gain
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Message started by raja.cedt on Sep 3^rd, 2014, 3:39am

Title: Passive circuit voltage gain
Post by raja.cedt on Sep 3^rd, 2014, 3:39am

Dear all,
While reading Fast analytical techniques for
electrical and electronic circuits book I found the attached passive circuit with voltage transfer function >1(1.12). I still didn't get the point, how can passive circuit gives voltage gain. Can any one explain me the reason.

Thanks,
Raj.

Title: Re: Passive circuit voltage gain
Post by aaron_do on Sep 3^rd, 2014, 4:33am

A passive circuit can get voltage gain by a change in impedance. Conservation of energy is what's important. A transformer or LC network would do the job.

Aaron

Title: Re: Passive circuit voltage gain
Post by loose-electron on Sep 3^rd, 2014, 2:58pm

the circuit drawn has a DC supply, so for that the C's fall out of the operating point.

Many circuits without amplifiers can give gain (reactive elements, switched capacitors, etc)

purely DC? that means the LC elements are all invisible, so I don't see that.

Title: Re: Passive circuit voltage gain
Post by raja.cedt on Sep 3^rd, 2014, 3:38pm

@Aaron: Yes I know, we have to consider to energy conservation rather voltage gain. Till now I haven't come across any RC circuit with voltage gain>1

@loose-electron: no it's about some intermediate frequency. Can you give me one RC example. The reason why I am asking about this is, many LC networks provide gain >1 but these gain could be V/I or I/V but I haven't seen any Rc,LC ckt with V/V or I/I>1...

Thanks,
Raj!

Title: Re: Passive circuit voltage gain
Post by RobG on Sep 3^rd, 2014, 3:53pm

I can't remember how it actually works, but if the capacitance is voltage dependent it can be used to create gain. See http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=1234357&url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F8736%2F27661%2F01234357.pdf%3Farnumber%3D1234357

Title: Re: Passive circuit voltage gain
Post by aaron_do on Sep 3^rd, 2014, 5:03pm

Hi,

If you include active elements like diodes or switches then there are other ways to get voltage gain. For purely passive switches, I don' know of any circuit which can give voltage gain at DC.

A transformer obviously can give voltage gain. I think you may have some confusion about LC networks (just read about the L-match) because they can also readily be designed with voltage gain (not just V/I or I/V). For RC, the amount of voltage gain which can practically be achieved might be very limited since they are inherently lossy (unlike LC networks) which is probably why you don't see them. I've never tried this circuit before, I'm just assuming the author wasn't lying.

Aaron

P.S. I just came across this article online called, "AC Voltage gain using just resistors and capacitors?" by Kendall Castor-Perry. You might want to google it...

Title: Re: Passive circuit voltage gain
Post by raja.cedt on Sep 4^th, 2014, 2:24am

Dear Aron,
I still some how not convinced how it can give >1 gain (intuitively), any how I have to sit and think properly. But thanks for the excellent doc.

Raj!

Title: Re: Passive circuit voltage gain
Post by Lex on Sep 4^th, 2014, 2:53am

I guess the text is about the zeros in the transfer function. ;)

Title: Re: Passive circuit voltage gain
Post by AnilReddy on Sep 17^th, 2014, 2:17am

Raja,

A quick analysis of the transfer function revealed it has a pole and a zero. For C1=C2 & R1=R2 case....zero occurs in front of the pole, which means, magnitude rises (>1) with frequency for sometime.

Bye.