I'm trying to build (in pspice first) a system to detect a modulated signal using an envelope detector. I'm first using the MULT part to act as the 'mixer' and then use a BJT amplifier to increase the signal voltage (I tried using a regular OPAMP but to no avail). I'm getting the amplifier signal fine, but when connecting the output to the envelope detector, the amplifier signal becomes very small. I tried putting a capacitor in series with the envelope detector which does return it to the original voltage level but then the peak-to-peak voltage is very small. Either way, the output at the envelope detector is useless. Is there something I'm missing here?

Thanks in advance.

Edit: here's the circuit
http://img69.imageshack.us/img69/1730/picture1nu0.png

 

It's because you are using a mixer instead of an AM modulator. So instead of a modulated signal, you are just getting two CW signals of 780Khz and 760khz. There is nothing in the "envelopes" so to speak. Another thing: the DC bias for the 2N3904 looks all wrong. The transistor will be in saturation with the way you've biased it. You also need a resistor to ground at the junction of C3 & D4.

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The base voltage of the transistor is too high, the collector resistor's value is much too high, the load resistor's value is much too low and the supply voltage is too low.

The transistor is severely saturated, overloaded and can't amplify.

Using a 9V supply, bias the base at about 1.1V. Then the emitter voltage is 0.5V. With a 1k emitter resistor the emitter current is 0.5mA and a 7.5k collector resistor will set the collector voltage at 5.25V where it can swing up to 9V and down to 1.1V. Connect a capacitor across the emitter resistor for much more voltage gain. Increase the value of the load resistor to about 5 times the collector resistor's value.

Attached Images

demodulator.PNG (14.8 KB, 20 views)

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Uncle $crooge

 

I'm sorry but can you explain the purpose of the resistor to ground betweem C3 and the diode?

Thanks guys for all your help!

 

I figure i'd ask another quick question since this thread is still somewhat recent. I was unable to implement an AC amplifier using an op-amp. My attempt is below. What am I doing wrong?

http://img210.imageshack.us/img210/4206/picture1zf5.png

 

With the diode in series with C3, C3 can only charge. It can never discharge. ie: You have a capacitor which can only pass AC in series with a diode which can only pass DC. The resistor creates a discharge path to ground for C3 during the negative half of the AC cycle when the diode is reverse biased.

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That's because it's a buffer, NOT an amplifier.

Simply google for "741 tutorial", there's LOT'S of infomation out there - opamps are probably the simplest components to use.

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The lousy old 741 opamp cannot swing its output the full amount above only 9kHz. At 700kHz its max output is only about 50mV peak-to-peak.

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Uncle $crooge

 

can anyone point me to a website that has the circuit model for an OPAMP ac amplifier. I just tried implementing one I found online today but was not getting anything. my friend said i have to build a voltage divider at the input of the opamp but was not very specific.

thanks in advance.

 

There are thousands of sites that explain how to bias the input of an opamp with 1 resistor and setup negative feedback using 2 resistors for it to have a high input impedance and any amount of gain that is needed at low frequencies.
In Google enter Opamp Tutorial or Opamp theory.

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Uncle $crooge

 

The input signal i am using is an AM modulated signal with a 750kHz carrier and 10KHz message signal. Should I just use a different type of opamp or is it still possible to use the LM741. I've simulated many different circuit configurations in PSPICE and keep getting the clipped output.

 

If you want to use a legacy OpAmp then the LM318 would be a better choice than the LM741 though you'll only have a gain of around 25db at 1Mhz with the LM318. For more performance, you could try an OpAmp like the OPA356 by TI. It's a simulation, so cost shouldn't be an issue.
Edit:
I've modified your diagram so the OpAmp has a gain of 10 instead of infinity(well in theory):

Attached Images

picture1zf5.png (9.6 KB, 9 views)

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Did you see in the datasheet for the 741 that its output drops above only 9kHz?

Did you see in the datasheet for the LM318 that its output is fine up to 1MHz?

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Uncle $crooge