# Voltage Amplifier Stage (VAS) design

Discussion in 'General Electronics Discussion' started by Le Sy Hau, Jul 20, 2015.

1. ### Le Sy Hau

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Jul 20, 2015
Can you guys help me design the Voltage Amplifier Stage in the picture I've attached. In this case, we know Vcc (dual supply), current source IB, and the value of V1 when no input signal.

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2. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Who is "we"? The schematic doesn't show us Vcc or any component values. I don't think anyone here can answer your question without more data.

Chris

3. ### Le Sy Hau

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Jul 20, 2015
I think if anyone who know about this circuit, they don't need the value of components because they can find the way by using virtual value, example: if R = 1, Vcc = 10, so I should be equal to 10/1 = 10A. But in this case, we can use I = Vcc/R instead of the real value. That is the way I learn Electronics. I know it maybe hard to find out the answer without component value, but I need to understand why we choose this value or that value for a component, I don't wanna here the sound like: we choose this value for this resistor without reason. But any way, thank you.

4. ### duke37

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Jan 9, 2011
This looks like a common audio amplifier output stage. To start, you need to decide on the output power required, this will give an idea of the output resistance. Then you need to decide on the current and voltage in the output transistors and the standing current to reduce crossover distortion.
So work through to the input. Note that the DC levels will have to be set, propably by negative feedback from the output to the input.

The load should be driven through a capacitor.
Do you understand the operation of each component?

5. ### Le Sy Hau

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Jul 20, 2015
OK, lets me say this, I am designing a class AB audio amplifier, output power is 3W with 4 ohms load, so Vcc should be equal to 5V (I used dual supply for my circuit), I used 0.1 ohms emitter resistor to minimize crossover distortion, 2 power transistor are Q2SD882 and Q2SB772, I have chosen quiescent current through two emitter resistor is 1mA, and find out the value of Vbe multiplier's components, 16.5mA current source (IB) and set DC level at V1 point when no signal is 0.615V. That's it. I simulated this circuit using PSpice, I put a signal into V1 point, with DC level is 0.615V, signal frequency is 1kHz and 8V peak-peak.
1st picture is output voltage swing (max 4V)
2nd picture is output power swing (max 3W)
3rd picture is my circuit but it is still unfinished
4th picture is DC voltage bias
5th picture is DC current bias (I designed I quiescent current is 1mA but I don't know why its become 6mA.

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6. ### Le Sy Hau

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Jul 20, 2015
So, hope you guys can help me design VAS stage for this circuit

7. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Well yes, now you're posting data. Your first post didn't include anything.. No Vcc, or constant (Q15) current, etc.

Chris

8. ### Le Sy Hau

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Jul 20, 2015
yeah. and if you need more data, just ask me. Thank you for your repply

9. ### Le Sy Hau

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Jul 20, 2015
There is no one can help me?

10. ### Alec_t

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Jul 7, 2015
The bias currents and voltages will be very temperature dependent (which is why negative DC feedback is used conventionally, to counter that). I'm not familiar with ORCAD; do you have to specify temperature?
BTW, unless Q12/15/17 are very closely matched and maintained at the same temperature it will be difficult to get predictable current mirror effects.

11. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Those are very good points Alec but I'm not familiar with Orcad either. Tina (which I use) doesn't default to including temp in their sims but it does provide a temp analysis.

Chris

12. ### Le Sy Hau

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Jul 20, 2015
Yeah now I have understood why it's become 6mA, because in practice, the dc current gain of transistor is not constant, but I used it as a constant. And I'm designing a open-loop circuit, when I'm done with this, I will add a feedback circuit. And one thing you guys should to know that OrCAD can analyze the circuit with temperature as well.

13. ### Le Sy Hau

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Jul 20, 2015
I had looking for some thing, or someone in this website can help me, but honestly there is no one can help me design a SIMPLE, very SIMPLE VAS circuit. So sad... I'm just 3rd year university student, I'm not a professional, but I think I can do it, and I'm going to finish this stuck. But any way, thanks for all of you guys.

14. ### Martaine2005

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May 12, 2015

As far as I can tell, the kind members on this forum ARE helping you.
They will NOT design it for you!!
If you have questions, ask them. When you don't understand something, ask a question.
DON'T sit there and wait for some miracle circuit designed by somebody else.

Just my 2 pennies-worth

Martin

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15. ### Le Sy Hau

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Jul 20, 2015
Of course, I am not waiting for someone who going to designs it for me, I am waiting for a suggestion, like "you need to based on this things to calculate that things, or some thing else about VAS stage, etc..., to find these component value. But clearly I don't see any suggest about VAS stage.

16. ### Alec_t

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Jul 7, 2015
Ok then. In your simulation, try doing a temperature analysis, try using transistors with different current gain specs, try using DC feedback to stabilise the operating points. How much effect do these changes have? Are variations acceptable? What is the power dissipation in each component? These are all experiments/determinations under your control and should give a good indication of component values/ratings needed. There isn't a magic formula for design .

Martaine2005 likes this.
17. ### Arouse1973Adam

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Dec 18, 2013
What about the power amplifier output stage, do you need that explained also? I find it very strange that you have the input and output stage but not the bit in the middle.
Adam

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18. ### CDRIVEHauling 10' pipe on a Trek Shift3

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May 8, 2012
Adam, I haven't been following this thread closely but judging by his schematic I think Q1, Q2 are the output stages and R16 (4Ω) is the speaker load. Then again it's Sat afternoon (raining) here and I'm doing my best to support the grape growers of the world. So, I may read this tomorrow and think...Huh?

Chris

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19. ### Martaine2005

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May 12, 2015
Ha, I am supporting the Belgiuns...

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20. ### Arouse1973Adam

5,164
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Dec 18, 2013
Chris you crack me up, I am enjoying the apple growers (cider) so yes lets look at it later. Nice comment though he made about no one on here can help him! I can think of half a dozen people, me and you included that could piss this.
Adam

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