Low gain audio stage - UK Vintage Radio Repair and Restoration Discussion Forum

Neil Purling · 8th Mar 2016, 9:32 pm

I want a single-transistor stage that would give a gain of 2-4 times, working from 9 volts.
It is as a pre-amplification stage in order to get a total gain of 100 or 200 from a TDA2822.
I bought a TDA2822 circuit and I know the data-sheet gives the gain as 50 only, which is not enough. The intent being to substitute the TDA2822 circuit in a role where a LM386 is utilised.

8th Mar 2016, 9:44 pm

How about feeding a weeny bit of the output back into the input? OK it might go unstable but worth a try. Or it's the classic common emitter circuit with the emitter resistor 1/2 to 1/4 of the collector one, plus bias of course. See attached quick idea...

8th Mar 2016, 9:46 pm

Done in 'fuzzy cad'!

mhennessy · 8th Mar 2016, 9:52 pm

To save having to draw it myself, take a look at the first schematic here: http://www.electronics-tutorials.ws/...ier/amp_8.html

Try making R1 47k, R2 10k, RE 1k and RL 4k7

Don't fit C2

C1 could be 1uF or thereabouts, and you'll probably need a capacitor at the output - the value depends on what follows it, and how low in frequency you need to go...

In theory, the gain of the circuit is RL divided by RE, which will be 4.7 here. You could decrease RL to 3k9 or similar as needed.

I would consider decoupling the power supply rail - perhaps put 100 ohms between the amp and the supply, and put 100uF or similar between the top of RL (and R1) and ground.

Input impedance - if it matters - will be about 8k.

This circuit won't be the best in the world, but it'll do (and will be a useful learning experience). Built it up, and check the DC voltage at the collector - hopefully it'll be somewhere near to 4.5V. If so, the audio signal should go through with no problems.

Good luck,

Mark

8th Mar 2016, 9:32 pm	#1
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Low gain audio stage I want a single-transistor stage that would give a gain of 2-4 times, working from 9 volts. It is as a pre-amplification stage in order to get a total gain of 100 or 200 from a TDA2822. I bought a TDA2822 circuit and I know the data-sheet gives the gain as 50 only, which is not enough. The intent being to substitute the TDA2822 circuit in a role where a LM386 is utilised.

8th Mar 2016, 9:44 pm	#2
Guest Guest Posts: n/a	Re: Low gain audio stage How about feeding a weeny bit of the output back into the input? OK it might go unstable but worth a try. Or it's the classic common emitter circuit with the emitter resistor 1/2 to 1/4 of the collector one, plus bias of course. See attached quick idea... Attached Thumbnails

8th Mar 2016, 9:46 pm	#3
Guest Guest Posts: n/a	Re: Low gain audio stage Done in 'fuzzy cad'!

8th Mar 2016, 9:52 pm	#4
mhennessy Dekatron Join Date: Jul 2007 Location: Evesham, Worcestershire, UK. Posts: 4,244	Re: Low gain audio stage To save having to draw it myself, take a look at the first schematic here: http://www.electronics-tutorials.ws/...ier/amp_8.html Try making R1 47k, R2 10k, RE 1k and RL 4k7 Don't fit C2 C1 could be 1uF or thereabouts, and you'll probably need a capacitor at the output - the value depends on what follows it, and how low in frequency you need to go... In theory, the gain of the circuit is RL divided by RE, which will be 4.7 here. You could decrease RL to 3k9 or similar as needed. I would consider decoupling the power supply rail - perhaps put 100 ohms between the amp and the supply, and put 100uF or similar between the top of RL (and R1) and ground. Input impedance - if it matters - will be about 8k. This circuit won't be the best in the world, but it'll do (and will be a useful learning experience). Built it up, and check the DC voltage at the collector - hopefully it'll be somewhere near to 4.5V. If so, the audio signal should go through with no problems. Good luck, Mark