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Amplifying with transistor

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Advanced Member level 4
Jul 27, 2007
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One circuit creates ultrasound signal through a pipe (to determine whether pipe contains liquid or not, I had liquid in the pipe already), a detector convert that sound to a very small signal (continuous pulse), so it has a amplifying circuit

I draw the circuit here, that pulse acrossed the filter (with dc offset about 1.7Vfrom another source) to the base of Q1.

When I put the probe of the oscilloscope at input location (left of C1), see nothing (because it is very small), but why I see that pulse to the base of Q1 ???

Using oscilloscope I get at the collector of Q1 the same signal (I think it must be pulse with higher amplitude so when pass by C3, DC lost, the amplitude still high enough to activate Q2)
and the result is pulse via C3 lose dc offset and its amplitude is below 0.6V then Q2 doesn't works, so it stop here and microcontroller just only think that the pipe contained no liquid and doesn't allow machine to work.

All components is SMD type, do you know where the wrong is ?
Thanks so much !!!

why are u using transistors when u can do it with op-amps only. for transistor you have to bias the circuit at its Q point. and its gain is not so high u better use an op-amp for amplification and then a comparator for use in digital circuit.

On the right circuit side, the signal is shorted to VCC.
The left side can work, but did you tune the LC circuit?

That is the designed circuit for the machine already, I open the machine and see that circuit.
Yes, I don't know what is the role of LC circuit here, just think it only needs a resistor to amplify the amplitude of the input pulse (at least above 0.6V)
Should I add something to fixed it, board uses SMD components, hard to modify somewhat ?

I have gone through the circuit the first stage is Class C amplifier has the ability to amplify the signal in pulse mode also it increases the power of the signal.

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