Amplitude Modulation using Op Amp.

[Miguel Lopez]

Hello.

I've trying to make an amplitude modulator using Op Amps, both I didn't get it. I'm asking for help on an schematic to do this.

I used the adder configuration with op amp and even when the carrier contained the modulator signal, it is not exactly what I'm looking for: a true Amplitude Modulator

Any help welcomed

Thanks

PS: Please, only op amp or maybe bipolars. I don't have any FET and I won't be able to get them.

 

[BobK]

Let me google that for you:

http://www.lmgtfy.com/?q=op+amp+amplitude+modulation

Bob

 

[Miguel Lopez]

Thanks Bob

Google and you are both good friends

 

[Miguel Lopez]

Sorry but all schems found in that search use FET's or the CA3080, which is to available to me.

Thanks anyway.

 

[BobK]

Interesting, you are indeed right, they all seem to use a jfet as a voltage controlled resistor. I know I have found some discrete transistor cicuits based on a differential amp state, but I can't seem to find one now, and none using op amps only either.

Bob

 

[(*steve*)]

Sorry but all schems found in that search use FET's or the CA3080, which is to available to me.

Miguel, PM me a mailing address and I'll see what I can do.

Are you comfortable with smd devices?

 

[Miguel Lopez]

I was trying this circuit on Electronics Workbench 5.1 sofware and althought it seems to modulate, positive halfcycle of the modulated signal are higher than negative ones.

What am I doing wrong?

I've been trying a Gilbert cell too but I have not reached positive results neither.

 

[CDRIVE]

I was trying this circuit on Electronics Workbench 5.1 sofware and althought it seems to modulate, positive halfcycle of the modulated signal are higher than negative ones.

What am I doing wrong?

I've been trying a Gilbert cell too but I have not reached positive results neither.

You're FET has no negative gate bias. You need a source resistor to bias the FET in its linear region. As it stands, your FET is will not transfer the positive excursions on the gate to the Drain.

Oops! Never mind. I didn't see the neg bias input. Must rethink...

Chris

 

[CDRIVE]

Please post the FET & OpAmp part numbers.

Chris

 

[Miguel Lopez]

Please post the FET & OpAmp part numbers

.
Generic.

Hey..come on. I have never worked with a (word with "F", ending with "-ing") FET. I'm trying to learn and I'm asking for help.

You're FET has no negative gate bias. You need a source resistor to bias the FET in its linear region. As it stands, your FET is will not transfer the positive excursions on the gate to the Drain.

A diagram would be nice.

Thanks

 

[(*steve*)]

.I have never worked with a (word with "F", ending with "-ing") FET

I understand your frustration.

CDRIVE points out that the problem could be that the FET is not in a linear region.

The transfer characteristics of gate voltage to channel resistance are non-linear (which will always produce some amount of non-linearity) but the trick is to us a small region where it approximates a straight line.

Your problem lay be that you are driving the input with too large a signal, or that the bias voltage is too low.

Since you are simulating at this point, I would try different combinations of input signal amplitude and bias voltage. This should show the effect of these.

I'm not sure you'll ever get 100% modulation with this circuit, and in any case, the relative amplification is not linear with changes in the mosfet's channel resistance. There is way more scope on the up-side than the down side for changes in gain.

What you essentially have here is a voltage controlled amplifier. Googling those *may* give you some pointers on how to improve linearity.

I think this circuit is more of a demonstration of an effect than a practical implementation.

Sorry I can't be of more help.

 

[(*steve*)]

This google search gives some useful results.

There are options with 2 op-amps, and another with 2 jfets.

Both would be interesting for you to simulate.

What frequency do you want this to operate at by the way?

 

[CDRIVE]

.
Generic.

Hey..come on. I have never worked with a (word with "F", ending with "-ing") FET. I'm trying to learn and I'm asking for help.


A diagram would be nice.

Thanks

Please rephrase your statement, as I'm not sure what you're trying to convey in the large print.

 

[(*steve*)]

Please rephrase your statement, as I'm not sure what you're trying to convey in the large print.

I'm sure the word is F(rustrat)ing.

Although there are a few others that come to mind. (facilitating, failing, falsifying, faltering, fascinating, faultfinding, fecundating, fenagling, festering, fetching, fibbing, fictionalising, fiddling, fidgeting, filching, finicking, fitting, flamming, flapping, flimflamming, flipping, floundering, fluctuating, flummoxing, flunking, flustering, foiling, forboding, forgiving, fornicating, fouling, foundering, foxing, fragging, fraying, frazzling, freaking, freethinking, fretting, frigging, frightening, frolicking, frothing, fuddling, fudging, fugging, fulfilling, fulminating, fumbling, functioning, funking, fussing, and of course f...ing)

 

[CDRIVE]

Thanks Steve. It was a rhetorical question.

Quote (Judge Judy): "I was born at night but not last night"!

Chris

 

[Miguel Lopez]

What frequency do you want this to operate at by the way?

MW range approx. 500kHz to 1600kHz would be nice.

Thank you very much for the Googling.

And thank you for the words too. I didn't know so many words in English with such characteristics.

 

[Miguel Lopez]

I did it with the Gilbert cell !!!!.

I used 2N2222 and 2N3904 transistors, but surely there should be any more suitable one for this purpose. I have some of them and they are in the EWB5.1 library.

Two issues about this circuit.

1.-) I had to use 48V DC for the Gilbert cell (not exactly a problem)
2.-) Carrier signal is not exactly a sine-wave in the output. It is a bit distorted. (would that be a problem testing a radio set?) See carrier distortion on second picture.

 

[(*steve*)]

The distortion of your sine wave looks more like an artefact caused by sampling than an actual distortion.

(I think I went through about 500 f...ing words to come up with that list. I picked only the ones that seemed to apply. Yes, I too was surprised at the number of words.)

That circuit uses a number of differential pairs. To work well, these need to be well matched and tightly coupled transistors. You may be able to get away with physically holding the two transistors together (or mounting them close together on the same heatsink) but for better performance you need real differential pairs. Something like an LM3046 (which I only pick because I have some -- there may be many others) could be the way to go. (but I understand the difficulties in obtaining them)

The operation in practice *may* be less than perfect. But it is still worth trying (and appears to be a good solution).

Whilst your selection of transistors is not perfect, at the frequencies you are contemplating they should perform. Both (from memory) have a transition frequency way above 2MHz (I think it is 150 to 300MHz-ish)

 

[CDRIVE]

Miguel, what power output to you want this transmitter to have?

Chris

 

[Miguel Lopez]

Transistors are not a problem. With the 1T311 it would work properly. Saddly the LM3046 is not an option due to availability. I was trying Op Amps in the EWB5.1 but none of which I have at hand can work at such frequencies, so I decided to test transistors.

Miguel, what power output to you want this transmitter to have?

It is not for a transmitter, just to have a tool to test radios. So, power is not a problem. It will only be use on my bench. Some day I want to built my own radio, as a hobby. If I can not built a superhet, I'm gonna try a regen at least.

I have never get a real AM signal. I have built circuits which just add the carrier and the modulating waves, and althought the resulting wave has the modulating wave in both edges (in phase instead of counterphase as in real AM), I don't know why this kind of solution is not used in radio. Anyway, AM is a challenge that I have never achieved. I think I'm getting closer.

PS. (Off-topic) I,ve been trying JFETs in the EWB5.1 and I'm making progress. Not to be used in this circuit specifically but for amplifying purposes in general. I'm learning something new.