Triode gain reduction / bias

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vmanj

Well-known member
Joined
Oct 3, 2017
Messages
297
Greetings to all.

In image 1, the standard triode inclusion.
In my microphone preamplifier, a circuit is applied as in image 2.

I have a few questions.

- If I understand correctly - R4 added to the circuit to reduce the gain of the stage?

- Cathode bias (hot / cold) is adjusted by resistor R3.
Does R4 have a similar effect on cathode bias, or does its magnitude solely affect the decrease in gain?
 

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The bias is set by the current flowing in R3 which is the same value in both cases. The additional 10K resistor introduces negative feedback which reduces the gain. In this case the gain is roughly the 100K plate resistor divided by the additional 10K resistor.

Cheers

Ian
 
Thank !

For example, if R3 - 1K is the so-called “warm bias”, in option 1, then it will remain “warm” in option 2, but with less gain.

I hope I understand you correctly ?
 
Basically yes but I don't know where you get the idea that 1K is a 'warm' option whatever that means. The quiescent (bias)  current in the tube is held more or less constant by the dc negative feedback due to the 1K resistor. The actual bias current depends on the resistor value but also on the plate voltage.

Cheers

Ian
 
Generally speaking, decreasing the value of the plate resistor, or increasing the value of the cathode resistor will result in a decrease in gain.
 
vmanj said:
- If I understand correctly - R4 added to the circuit to reduce the gain of the stage? 
Yes, but the main purpose is to provide an input for negative feedback.

- Cathode bias (hot / cold) is adjusted by resistor R3.
Correct.

Does R4 have a similar effect on cathode bias, or does its magnitude solely affect the decrease in gain?
It changes the bias point only a few percent, mainly because the voltage across the tube decreases a little.
But remember that the input signal must be DC decoupled (or floating) because the grid sits at about 10V above ground.
 
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