Created by
Created November 13, 2011
Last modified June 02, 2013
Tags amplifier   audio   bjt   filter   frequency-domain   high-pass   low-pass  

Summary

A one-transistor amplifier, intended to amplify audio picked up by a piezoelectric transducer.


Description

This models the amplifier circuit described in detail at http://www.nerdkits.com/videos/sound_meter/ .

Vpiezo and Cpiezo in series represent a piezoelectric element that's being vibrated mechanically by sound waves, and is generating a voltage as a result. R1 and R2 represent a 100K potentiometer. Together with RE, these set up a 100uA bias collector current for transistor Q1. CE provides extra amplification.

A few things to try in CircuitLab

This circuit highlights several cool features of CircuitLab simulations that you can quickly try. Load this circuit with the "Open in editor" button above, and then click the "Simulate" tab to get started.

DC

Run the DC simulation to quickly get a sense of the bias currents, etc.

DC Sweep

See how the circuit varies of our power supply isn't really at 5V.

Time

Take a look at how a sine wave input looks, and how distorted it is at the output. You can adjust the amplitude of the sine wave source (Vpiezo) and re-run to see how distortion varies with amplitude.

Bode

Run the frequency-domain analysis to see a plot of the gain of the amplifier. As-is, it's about +38dB in the passband. That's a voltage gain of 10^(38/20) = 79. Notice that our hand calculations indicated a gain of about 88, so that's pretty close!

Now remove CE, and re-run the Bode analysis. The gain is now about +15.7dB, or a voltage gain of 10^(15.7/20) = 6.1 That's pretty close to the 6.7 we calculated by hand.

More Common Emitter Amplifier info

  • Take a look at the third video here for another example about simulating a common emitter amplifier

Comments

I have a few questions that I'm hoping you can answer.

1) why us a capacitor instead of placing a larger resistor in for RE?

2) in other examples I have seen, people are connecting this to an op-amp. Why would someone want to do that?

3) is there significant concern for outside interference, and if so, how would you rectify that

Thanks for the advice

by pdebrois
March 11, 2013

1- the capacitor is to provide increased gain for all but the lowest frequencies. Look up bypass capacitor or try removing the C and see what happens to the output 2- An op amp will have extremely low output impedance, also they could get more gain 3- the signal is large so not much of a problem you could build he circuit in a metal box and shield it

by melanfred
April 26, 2013

If i want to design Audio Amp. to deliver an avg. power of .15 to an 8 ohm speaker from microphone that produce 10 mw peak sunsoidle signal asourse resistor of 10 kohm ????

by demaabudaboos
July 17, 2013

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