Author Topic: coupling capacitor  (Read 1598 times)

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Offline yalectTopic starter

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coupling capacitor
« on: July 11, 2017, 10:50:01 am »
Hello,
I would like to ask you I want to choose capacitor for transistor amplifier input and output.
please, can you led to me how I can do that or what's formulas and conditions?
Thank you
 

Offline Benta

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Re: coupling capacitor
« Reply #1 on: July 11, 2017, 12:00:43 pm »
For the input cap, use the input impedance and calculate the CR divider for a reasonable cut-off frequency, say 20 Hz.

For the output, make an assumption about the minimum impedance of the load and do the same calculation.

 

Offline yalectTopic starter

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Re: coupling capacitor
« Reply #2 on: July 11, 2017, 11:10:37 pm »
 Hello, thank you for your reply
what do you mean by the CR divider for a reasonable cut-off frequency, or can you give formula or example?
Thanks
 

Offline Paul Moir

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Re: coupling capacitor
« Reply #3 on: July 12, 2017, 12:39:59 am »
Commonly called a "High pass RC filter" or maybe a "first order passive high pass RC filter.  The R is the input impedance of your next stage, while the C is your coupling capacitor. 
Lots of good info out there, such as:
http://www.electronics-tutorials.ws/filter/filter_3.html

With the corner frequency Fc=1/(2*pi*R*C) being the one of interest to you.
 

Offline yalectTopic starter

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Re: coupling capacitor
« Reply #4 on: July 12, 2017, 11:00:32 am »
Hi,
thank you for your reply but what's a simplest way to determine The appropriate value of coupling capcitor (input and output) ? considering the input impedance of the amplifier if we know that Extracting that impedance is something complicatedand.
 thank you
 

Offline Paul Moir

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Re: coupling capacitor
« Reply #5 on: July 13, 2017, 04:40:13 am »
Well, if you can't compute it or estimate it, you can only measure it or simulate it with something like LTSpice.  I would think it would be fairly trivial to measure it by inputting a signal from a signal generator via a resistor, and measuring the signal on either side of the resistor with a multimeter.  Just make sure the frequency of the signal is within the multimeter's AC volt frequency limit.
 


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