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Old 1st December 2012, 03:28 PM   #1
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Default How to find the transfer function of this volume control?

what is the transfer function of this circuit? R3 and R4 are a potentiometer divided into two resistors.
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Old 1st December 2012, 03:46 PM   #2
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R4 is providing in effect negative feedback, because in the loop is an inverting amplifier.
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Old 1st December 2012, 03:47 PM   #3
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Let's say Vi is the input voltage and Vo the output voltage. Let's pray my knowledge of circuit theory is OK.
In such a case: Vo/Vi=-R4/{[R1/(1/R2+jwC2)](1/R5+1/R4+1/R3)-1/R3}
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Old 1st December 2012, 04:10 PM   #4
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i've simulated your transfer function and it is not right because it should give with the values in the photo -2.63dB which is a gain of 0.738 V/V, with your formula i have a gain of about 13 V/V
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Old 1st December 2012, 06:10 PM   #5
DF96 is offline DF96  England
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The title asks how to find the function. The text ask what it is. I will answer the first question. Circuit theory plus some algebra.
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Old 1st December 2012, 06:27 PM   #6
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i now what a transfer function is but i don't know how to calculate the transfer function of this circuit
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Old 1st December 2012, 06:59 PM   #7
DF96 is offline DF96  England
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Where did you find the circuit? What is your application? Please assure us that this is not a student assignment.
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Old 1st December 2012, 07:52 PM   #8
djoffe is offline djoffe  United States
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this is a variant of the Douglas Self/Baxandall high level gain stage circuit. It does a fairly good approximation of log gain with a linear pot. He uses essentially this in his latest preamp.

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Old 1st December 2012, 10:06 PM   #9
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The first time I derived the transfer function in a hurry. I checked it later and found a mistake.
Now it's my turn to ask. How did you know the mid-frequency gain was -2.63 dB?
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Old 2nd December 2012, 09:33 AM   #10
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i simulated it with LTspice but yesterday i calculated by myself the transfer function so now i have it and i don't need your help. thanks anyway
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