# Voltage amplifier with 2N3904

Discussion in 'The Projects Forum' started by yoannwyffels, Apr 12, 2016.

1. ### yoannwyffels Thread Starter New Member

Mar 23, 2016
16
1
Hello,

I've found a pretty good example:
http://www.instructables.com/id/Voltage-Amplifier/

For what I'm trying to do, I'm on a +5V DC Power supply (not a +15V), and need to have a gain x3
My input signal have a range between 0.5v and 1v

Does this schematic will be working well ?

Thank you for your help !

2. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,967
1,135
Are you talking about a DC signal or an AC one. If AC, do you mean a) it has an amplitude within the 0.5v to 1V range or b) it has a positive peak of 1V and a negative peak of 0.5V? If AC, what frequency?
Here's the result using your circuit with a 1V amplitude 1kHz AC signal. Do you see the problem and know the cure?

3. ### yoannwyffels Thread Starter New Member

Mar 23, 2016
16
1
Hello Alec_t,

I'm talking about a DC signal which go from +0.5v to +1v. Maybe this amplifier (with 2n3904) only work with AC signal ?

4. ### dl324 Distinguished Member

Mar 30, 2015
3,376
651
Would C1 pass any DC component of the input signal?

5. ### yoannwyffels Thread Starter New Member

Mar 23, 2016
16
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I think no, but even if I remove capacitor....

6. ### yoannwyffels Thread Starter New Member

Mar 23, 2016
16
1
Forget it, I will use any LM741 opamp to do what I want to do !

Dodgydave likes this.
7. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,967
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Even if you remove the capacitor the circuit shown won't work as you want with a ground-referenced 0.5V-1V input.
a) 0.5V would switch the transistor off,
b) taking the output at the transistor emitter gives no gain at all.
c) if you take the output from the transistor collector you get gain, but there is inversion, i.e. as the input voltage rises the output voltage falls.
When the input goes from 0.5V to 1V do you want an output going from 1.5V to 3V?
Good luck with that

8. ### dl324 Distinguished Member

Mar 30, 2015
3,376
651
With a 5V supply, you'll need a rail to rail opamp.

9. ### Dodgydave AAC Fanatic!

Jun 22, 2012
5,141
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Use an opamp like lm358, lm324

Mar 23, 2016
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11. ### dl324 Distinguished Member

Mar 30, 2015
3,376
651
Read the datasheet. Input and output voltage are only guaranteed to within 3V of the supplies and minimum supply voltage is spec'ed at 10V. It will likely operate at a lower supply voltage and, if you're lucky, you'll be able to find a unit that gives better than typical voltage swings.

Mar 23, 2016
16
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Mar 30, 2015
3,376
651

Sep 17, 2013
5,967
1,135

15. ### AnalogKid Distinguished Member

Aug 1, 2013
4,685
1,297
No, for two reasons. First, this is not a voltage amplifier, it is an emitter follower. This is a circuit configuration that has current gain but no voltage gain.

Second, while R1 and R2 might be the correct ratio of values for biasing the circuit, their absolute values are way too small and will load down the input signal.

ak

16. ### AnalogKid Distinguished Member

Aug 1, 2013
4,685
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Well, that didn't take long.

ak

17. ### yoannwyffels Thread Starter New Member

Mar 23, 2016
16
1
Yes AnalogKid, was trying to recycle some components I have in stock to do what I want, but when it's impossible,... it's impossible

18. ### Bordodynov Active Member

May 20, 2015
670
194
Maximum input signal (Vcc-Vce_sat)/2/Gain=(5V-0.4V)/2/3=0,766V

thomasbalshi likes this.