How does one select the values of components used in Amplitude Modulation Circuit (using a Transistor common emitter amplifier circuit)?

The circuit that I designed ends up giving a weird waveform but I am pretty sure my professor gave us the same circuit. Please point me to resources that can help me decide what values of components to use.

NB: I tried Google, but it gives me only theory and no guidance regarding the components in question. Also please note that I am new to electronics. Here is the circuit:

Edited to show better circuit diagram: -

The circuit I was given

Waveform that I get

  • \$\begingroup\$ The circuit I am talking about is this : encrypted-tbn1.gstatic.com/… \$\endgroup\$ – iluvthee07 Aug 30 '13 at 9:54
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    \$\begingroup\$ What does the weird waveform look like and what component values did you use? What carrier signal level and frequency did you try and what modulation signal did you choose? What type of modulation were you expecting? \$\endgroup\$ – Andy aka Aug 30 '13 at 10:19
  • \$\begingroup\$ Here is the circuit which our prof gave us : i39.tinypic.com/21kw0ug.jpg I expect AM waveform but the output is something like a complicated square wave \$\endgroup\$ – iluvthee07 Aug 30 '13 at 10:21
  • \$\begingroup\$ Try reducing the amplitudes of the carrier (1kHz) and modulation signal (10Hz) and, I'm a bit unsure about the position of the 10Hz - depending on the signal source it may give strange effect so, try grounding R3 (bypassing the 10Hz) and re-insert the 10Hz in series with C3. Also try carrier at 10kHz and modulating signal at 100Hz. I'd still like to see the waveform - can't you draw it dude? \$\endgroup\$ – Andy aka Aug 30 '13 at 10:34
  • \$\begingroup\$ @Andyaka : I have added the waveform also the edits in my circuit that you suggested did not help. The waveform hardly budges by some value. \$\endgroup\$ – iluvthee07 Aug 30 '13 at 10:50

Your biggest problem is C3. This capacitor has an impedance of about 47Ω @ 1000 Hz, which means that (ignoring V2 for the moment), the circuit has a gain of about -100 with respect to V1. With an input signal of 5V and a supply voltage of 15V, the output is going to be clipping pretty much all the time.

Try removing C3 altogether, and I think you'll find that the resulting output waveform is much closer to what you expect.

You may also need to tweak the bias resistor values. I think in this application, you need the transistor biased near cutoff, not at VCC/2 as you have it now.

  • \$\begingroup\$ On removing c3 I get something like an amplified sine wave but not amplitude modulation waveform. I will try tweaking the resistors as you suggested. Can you point me to some resources which can help me calculate what component values to use? \$\endgroup\$ – iluvthee07 Aug 30 '13 at 11:28
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    \$\begingroup\$ Show us the new waveform. Also, understand that this circuit isn't going to give you the picture-perfect AM waveform that you'd get by mathematically multiplying the two signals. It needs a narrowband filter at the carrier frequency at its output to remove unwanted distortion products. And obviously, the single best resource I can point you to is the professor who gave you the circuit in the first place. \$\endgroup\$ – Dave Tweed Aug 30 '13 at 11:48
  • \$\begingroup\$ which is the exact R2 and C2 value ? Also what is resistor twiking ? To drive transistor in cutoff ... which values of R4 and R5 are required ? \$\endgroup\$ – user39292 Mar 26 '14 at 9:44

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