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Basic Electronics » CE Amp design

November 10, 2010
by Hexorg
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Hey everyone! So, in my analog electronics class we just finished CE Amp analysis... and it seems easy enough to calculate gain when you have all the resistors (voltage divider biased). So I decided to implement CE amp in my project... and got in trouble... what resistors should I use?!

My signal source is going to be an RCA-output sound from my XBox. Somewhere online I found that maximum Vrms for RCA-plug sound is 775mV. Converting it to Vpp = 1.096V. I'd like to crank it up to 5Vpp, so gain should equal to 4.899, also it'd be great to "put" that signal on top of a 2.5VDC to have resulting voltages go from 0V to 5V with respect to ground.

The only available Vcc I have is 5V, so I prefer that to stay that way. I can get any transistors, resistors, and capacitors, so I can choose any. Also I plan to feed the output to the Analog-to-digital converter of a microcontroller. I looked in the datasheet, but couldn't find resistance of ADC, so i don't know RL, however I found that "The ADC is optimized for analog signals with an output impedance of approximately 10 kΩ or less."

Can you please give me some ideas to start with choosing resistors? I looked online for quite a while, found a bunch of CE amp analysis, but no where - design.

November 10, 2010
by mrobbins
(NerdKits Staff)

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Hi Hexorg,

As a start, did you see our NPN amplifier design as part of our Piezo Sound Meter video? It's a 20-minute video tutorial that's mostly about that.

At low frequencies, the input impedance of the ADC is practically infinite -- the ATmega168 datasheet on page 312, "ADC Characteristics", shows 100 Megaohms typical. You'll supply your own load resistor external to the ADC.


November 10, 2010
by mongo
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If you use AC coupling between stages, the amplifier is pretty simple. All you need to do is set your bias on the base to just above the point where the transistor starts to amplify. Use a high enough resistor/potentiometer value so an AC coupling capacitor can pass the signal to the base. Use another at the output (collector) to the next stage. If you use a resistor at the emitter, you can also add a R/C pair across it to give you an even higher AC gain without affecting the DC gain.

I have a couple of schematics to upload so I'll add this one to the list.

November 11, 2010
by Hexorg
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Mongo, thanks, but I'm trying to get a gain of only 4 so I don't think I need a multi-stage amp.

Mike, yes, I watched the video thanks! just what I needed :D

November 12, 2010
by BobaMosfet
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Go slow, consider everything. Your beginning to work with and design signal electronics. There are a lot of things to consider.

  • Impedance mismatch.
  • Input .v. Output Impedance.
  • Thermal characteristics & Power Dissipation
  • Gain & Q Point
  • Capacitance, Resistance, Reactance, and Induction
  • Phase
  • RMS which is not a PPV measurement

You could simply use a transistor (perhaps multistage), meeting your A, B, Q-Point and thermal characteristics for optimal output, and a simple voltage divider to elevate the base to 2.5V.

Apply the same thinking to use an amplifier. Non-inverting input, voltage divider to elevate input base, feedback loop.

Either way, determine the numbers, then pick the part (the most tedious part of circuit design is sourcing) that meets your criteria.


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