Low Voltage signal amplifier

Discussion in 'Analog & Mixed-Signal Design' started by murgui, Apr 15, 2018.

1. murgui Thread Starter New Member

Apr 15, 2018
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0
Hello gentlemen,

I have recently found this problem in a job offer example test:

In the internet you might find several step by step calculations like http://www.edutek.ltd.uk/Tutorial_Pages/Transistor_Amplifiers_Calcs.html
However, I can't manage to make them work with 3 V. For validating the design I use falstad.com/circuit. Might not be the best option but I think it should be enough for this easy task. The result is the amplification from +- 500 mV to kind of a sinewave with 2,2 V of maximum voltage and 850 mV of minimum voltage. You can see the circuit ad play with it here:

The eemitter resistor is really small, and I'm not using the emitter capacitor, which is guess that changes the gain depending on input frquency. However, the first goal is to have the circuit amplifying correctly, and then, add the last features. Do you have any idea or suggestion of what should be taken in consideration with a 3 V supply?

Regards.

2. panic mode Senior Member

Oct 10, 2011
1,645
459
do you know what kind of gain you would need? what kind of gain is possible with that supply (and no distortion)?

Mar 10, 2018
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4. murgui Thread Starter New Member

Apr 15, 2018
4
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Hello.

Panic mode, I want a 1.5 voltage gain.

Dana, I've read the documents and these are not different from those I found in the internet like the one I suggested in the OP. I think that this procedures have this rules of the thumb for Vrc and Vre that work fine for a 10 V supply but are not good enough with such a low voltage supply. As a 1 V approximation doesn't have a big impact in a 10 V supply, but is one third of the available supply voltage.

Regards

5. Bordodynov Well-Known Member

May 20, 2015
2,044
622
Your desire is not doable. The input amplitude is 1V, the output amplitude of the signal (at amplification is 1.5) and 1.5V, and this is the Peak-Peak voltage is 3 V, which is impossible when the amplifier is powered from 3 V!
Here is the scheme that provides the maximum approximation to your desires:

murgui likes this.
6. murgui Thread Starter New Member

Apr 15, 2018
4
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I see, thank you for the information. I guess that the lower limit has it cause in Vce(sat) of the transistor.

How about the thermal stability without Re and the problems related to the manufacturing characteristics of the transistor in your design?

Regards

May 20, 2015
2,044
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See

8. Bordodynov Well-Known Member

May 20, 2015
2,044
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Unfortunately, transistors (different types) have different Base-Emitter voltages and different current amplification. To obtain the maximum output voltage, a mode adjustment is required. This means that the voltage across the collector is adjusted to half the supply voltage. For small signals, adjustment is not necessary.

May 20, 2015
2,044
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See

10. murgui Thread Starter New Member

Apr 15, 2018
4
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Really impressive!! Thank you very much for you time. I will try to find the mathematical limit of voltage output.

Regards.

11. danadak Well-Known Member

Mar 10, 2018
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Could always use an LM10, that will go down to 1.2 Vcc in operation, although
that won't produce 3V out at that supply V.

Has good CM in and out. Plus it has an onboard Vref for general use.