NPN Transistor Amp not working!!?!?!

[ChristianMCA]

I have been trying to make a transistor amp for months and everytime I fail. I always hook them up with working components and they never work and I am getting pissed, please help me. I have the schematic here and I followed and it does not work! I have also put a picture of what I did on my breadboard.
I used a 3904 NPN Transistor, two 1k resistors, two 4.7k, and capacitors to connect the input to the circuit (not shown in pic)
I connected a 9v battery to the circuit with the red wire, white for my input and blue for my output

 

[davenn]

I have been trying to make a transistor amp for months and everytime I fail. I always hook them up with working components and they never work and I am getting pissed, please help me. I have the schematic here and I followed and it does not work! I have also put a picture of what I did on my breadboard.
I used a 3904 NPN Transistor, two 1k resistors, two 4.7k, and capacitors to connect the input to the circuit (not shown in pic)
I connected a 9v battery to the circuit with the red wire, white for my input and blue for my output

hi, Christian
welcome to EP

what are you feeding into it?
what is the output going to ?
do you have a scope so that you can compare the input and output signal levels ?


Dave

 

[ChristianMCA]

hi, Christian
welcome to EP

what are you feeding into it?
what is the output going to ?
do you have a scope so that you can compare the input and output signal levels ?


Dave

I am feeding audio from my iphone into it (I know the dangers of doing that) I am outputting to am 8 ohm bookshelf speaker. I do not have an oscillascope

 

[davenn]

I am feeding audio from my iphone into it (I know the dangers of doing that) I am outputting to am 8 ohm bookshelf speaker. I do not have an oscillascope

So, you are probably overdriving the poor lone transistor amp for a start. You need to work with a VERY small input signal
Am not sure if the output impedance of that circuit would suit a 8Ω speaker. The speaker may just be loading it down ....

some one else may comment on that

 

[LvW]

A load resistor of 8 Ohms? This cannot work.
In your circuit diagram you have forgotten to show us one of the most important parts: The load!
I suppose you know how such a load determines the gain? Remember: The transistor acts as a CURRENT source - and the output voltage is determined by the collector resistor in parallel with the load!
Recommendation: use another transistor stage in common-collector configuration (emitter follower).

 

[73's de Edd]

Sir ChristianMCA . . . . . . .

You just are expecting WAAAAAAY-WAAAAAY more out of that WAAAAAY simplistic circuit than is being possible.
The INPUT device probably has more power input capability than you could EVER expect to
get out of this added amps end output.
That 4.7 k collector load resistor could NEVER drive an 8 ohm speaker tied to it, to ground.
An absolute minimum variant would be to install a 1K to 8 ohm output transformer in place of the
R3 resistor . . . . .but then to find that still is being woefully short, of what you probably are expecting.
The R1-R2 input biasing resistor values ratio are heavily weighted to requiring an initially
higher level input.
The high R4 emitter resistor is not going to let ANY adequate power level through.
As for any aspirations of driving a speaker, the minimally possible circuit to try next would
be this circuit below.
Most past date power amplifiers were just a variant of beefing up of this basic circuit .

This came from the annuals of an at times contributor here.
Here is the circuit below and tons of related writeup at the given site.
Lots of leeway in transistor substitutions for those given transistors.

HOT LINK:

http://www.talkingelectronics.com/projects/TheTransistorAmplifier/TheTransistorAmplifier-P1.html

Drop waaaaaay doooooown to its FIG 54aa


73's de Edd

 

[BobK]

And even with the circuit 73'd de Ed posted, you will get about 1/4 W of audio output. Enough to make sound from the speaker that won't wake someone one in the next room.

Bob

 

[ChristianMCA]

Sir ChristianMCA . . . . . . .

You just are expecting WAAAAAAY-WAAAAAY more out of that WAAAAAY simplistic circuit than is being possible.
The INPUT device probably has more power input capability than you could EVER expect to
get out of this added amps end output.
That 4.7 k collector load resistor could NEVER drive an 8 ohm speaker tied to it, to ground.
An absolute minimum variant would be to install a 1K to 8 ohm output transformer in place of the
R3 resistor . . . . .but then to find that still is being woefully short, of what you probably are expecting.
The R1-R2 input biasing resistor values ratio are heavily weighted to requiring an initially
higher level input.
The high R4 emitter resistor is not going to let ANY adequate power level through.
As for any aspirations of driving a speaker, the minimally possible circuit to try next would
be this circuit below.
Most past date power amplifiers were just a variant of beefing up of this basic circuit .

This came from the annuals of an at times contributor here.
Here is the circuit below and tons of related writeup at the given site.
Lots of leeway in transistor substitutions for those given transistors.

HOT LINK:

http://www.talkingelectronics.com/projects/TheTransistorAmplifier/TheTransistorAmplifier-P1.html

Drop waaaaaay doooooown to its FIG 54aa


73's de Edd

I totally redesigned my amp and I got it to work!

 

[ChristianMCA]

And even with the circuit 73'd de Ed posted, you will get about 1/4 W of audio output. Enough to make sound from the speaker that won't wake someone one in the next room.

I redesign my circuit and got it to work.

 

[BobK]

Seriously? Can you show us?

Bob

 

[ChristianMCA]

The capacitor is what I hook up to the output for a filter and it sounds really good. Only one problem is that the 1k resistor gets really hot.

 

[Audioguru]

Don't you have a multimeter that can measure DC voltage?
On your original single transistor circuit, without the speaker, measure the voltage at the collector output. Is it 4.3V?
Then connect the 8 ohm speaker and measure the DC voltage again. Is it 0.022V?
Can a transistor work if its collector voltage is only 0.022V?

Why is the DC voltage so low? Maybe because the low resistance of the speaker is shorting the output to ground.

You said you redesigned the circuit and were asked to show the circuit (schematic) but you did not show the schematic. Here are simulations of your original circuit:

 

[73's de Edd]

.

Sir ChristianMCA . . . . . . .

but . . .but . . .BUT . . .BUT . . .BUT . . . . .a 1K resistor can not heat up . . . ." really hot" . . . . . from a 9VDC supply.
I am still only seeing a breadboard with one electrolytic, one small signal transistor and two resistors, one of which has its color coding blocked by a jumper lead and its shadow,
The other resistor appears ? to be a 13 ohm resistor . . .now that's possible to "warm" a 13 ohm resistor.
But for this circuit, no where . . .can any speaker driving capability, I see.
Is this just an empirical one transistor stage learning experience . . . . . or an actual dedicated need for a speaker driving amplifier ?

73's de Edd

 

[BobK]

Please show us a schematic of your "working" circuit.

Bob