Calculate the resistance values of Class A amplifier

Thread Starter

Doppler2902

Joined Sep 15, 2020
35
Hello, I have to calculate the resistance values for this Class A amplifier (R1, R2, R3, R4):

1647629364344.jpeg

So I know that voltage in the charge of 100 ohms (R9) is 6 Vrms and for calculate R1, R2, R3 and R4 I did the following:

1647629737882.png

But I don't know how to operate it because I have three unknows (R3, Av and Ie for re'). Some idea for solve this? Thanks
 

Audioguru again

Joined Oct 21, 2019
4,693
With an input of 5mV peak which is 3.54mV RMS and an output of 6V RMS then you need a voltage gain of 1695 times which is impossible with the simple circuit you show,

You need a preamp with a voltage gain of about 80 times plus a power amplifier with a voltage gain of 21.2 times.
They both need much more gain so that negative feedback can reduce the severe distortion.

What is R9? Its 100 ohms is much higher than a 4 or 8 ohms speaker. The low capacitance of C4 will not pass bass sounds to R9.
 

Thread Starter

Doppler2902

Joined Sep 15, 2020
35
With an input of 5mV peak which is 3.54mV RMS and an output of 6V RMS then you need a voltage gain of 1695 times which is impossible with the simple circuit you show,

You need a preamp with a voltage gain of about 80 times plus a power amplifier with a voltage gain of 21.2 times.
They both need much more gain so that negative feedback can reduce the severe distortion.

What is R9? Its 100 ohms is much higher than a 4 or 8 ohms speaker. The low capacitance of C4 will not pass bass sounds to R9.
Yes, what you say is true in the practical plane. But theoretically how can I calculate the resistors values for that parameters of gain?
 

Ramussons

Joined May 3, 2013
1,264
You have only 1 voltage amplifier stage here - Q1.
The rest is basically a current booster to feed a low value load like a 8 ohms speaker.

I would remove C3 and take the voltage gain of that stage as (R3||R6) / R4, ensuring that R1 and R2 are at least 10 times R4.

This is just a Thumb Rule calculation.
 

Thread Starter

Doppler2902

Joined Sep 15, 2020
35
You have only 1 voltage amplifier stage here - Q1.
The rest is basically a current booster to feed a low value load like a 8 ohms speaker.

I would remove C3 and take the voltage gain of that stage as (R3||R6) / R4, ensuring that R1 and R2 are at least 10 times R4.

This is just a Thumb Rule calculation.
Hi, what you mean is this?
1647708019547.png
 
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