This is supposed to be a basic audio amp. How many errors here? No battery decoupling, DC going through the speaker and what's going on with Q2, Q3? Will that even work - or am I missing something?

Sorry, title should read 'errors' not 'effors' but can't find a way to correct that.

(Mods, please change heading if possible. Thanks)

Janet (Yabba Rays)

 

Yes, it has DC current going through the speaker which is generally a no-no.

You are correct, Q2 and Q3 won't work as shown.
You either need a resistor from the collector of Q2 to V+, or Q3 needs to be a PNP.

 

MOD NOTE: Title changed as requested by TS. Moved to Homework Help.

 

Q2, Q3

are most likely the NPN darlington drawn . . . has it happens -- coz the replacement of upper one with PNP enables a current path through the base of then PNP Q3 whitch eighter saturates the least or emtys the battery fast or reduces the terminal voltage or ruins the Q3 or/and Q2

i considered the collector feedback for the input stage Q1 - because the application runs on a battery - say 2x 1.5V that changes it's terminal votage from 2.9V down to say 2.3V during battery discharge-/service- time
i considered using different polarity transistor for Q1or reversing rails×polarity for Q23 darlington (both emitters go to speaker)

the circuit looks like drawn by instructions at bad quality phone call ???

 

Which book did you get that from? Can you supply the "text" as well?

 

on a second thought i'm not so sure about what i just said
► it's the 3× bjt gain of 200³ = 8000000 -- eight million

i attempted something like this - an audio power amp -
and for a reason - i had to negative feedback - in principle -
Q2 and also Q3 e.g. both stages - to get un-distorted
near 1:1 voltage amplified signal ??? while not varying
the supply keeping it steady 3V

 

ok, i can't design transistor wide bands (fq. × feed voltage), but since we don't have anything here , here what i attempted


(just a blind shot not that i'd had anything speciffic in mind - and i didn't yest compute anything)
optimizing . . .


still hasn't computed anything . . .

 

Thanks for all the helpful replies.

The cct is from here:

http://www.newtoncbraga.com/index.php/component/content/article?id=613:bioprobe-pn006e

The book is so full of errors as to be almost useless to its intended readers. I think I should politely point this out to Newton C. Braga!

(Incidentally, you can find any web image using a reverse image search - it's very useful!)

 

There is not a lot wrong with the circuit, only the drawing of Q3 is incorrect.

BC558 is a PNP transistor. The emitter should be on the +ve side of the battery. The collector is connected to the loudspeaker.

You can replace Q1 and Q2 (BC548) with 2N3904.
Q3 (BC558) may be replaced with 2N3906.
I would choose an 8Ω speaker (or higher), not a 4Ω speaker.

I would decrease R1 from 1M5 to about 560k-1M for less distortion.
Otherwise, the circuit works fine on 3V supply.