amplifier

[mstechca]

I have a question.

I am using this circuit (which I made myself) as my UHF amplifier. It seems that certain resistors and capacitor combinations give better results than some others.

The question is, what capacitor and resistor values should I use for the unmarked components and why?

I want to be able to amplify a 190Mhz signal.

An attachment follows.

 

[Roff]

It appears that you are trying to detect and amplify modulation on your 190MHz signal. What is that speaker-looking thingy that is labeled "200Hz"?
That amplifier will never have the bandwidth to amplify 190MHz. The first stage may act as a detector for any low frequency (audio range) AM that is on an RF input, and the second and third stages will amplify the audio. That emitter follower (the last stage) is very strange. You would be better off replacing the NPN and base resistor with a 50 - 100 ohm resistor.
You have no selectivity, so any frequency RF that shows up on the input will be mixed and/or demodulated. Is there a selective RF preamp, or at least a tuned circuit, in front of this?
We can deal with R and C values later, if there is any point to it. :roll:

 

[mstechca]

It appears that you are trying to detect and amplify modulation on your 190MHz signal.

I'm trying to amplify the demodulated VHF signal.

What is that speaker-looking thingy that is labeled "200Hz"?

I forgot to remove the 200Hz. That is supposed to be an ordinary 8-ohm speaker. I was designing my circuit in Electronics Workbench after doing the real circuit so that you can see it.

That amplifier will never have the bandwidth to amplify 190MHz.

How can you determine bandwidth?

The first stage may act as a detector for any low frequency (audio range) AM that is on an RF input, and the second and third stages will amplify the audio.

I agree, but when I hook it up to my VHF detector (basically a homemade aircraft receiver), I want to be able to pick it up loud and clear. Thats why I designed this circuit. Its the resistor and capacitor values that are confusing me.

That emitter follower (the last stage) is very strange.

Its uncommon because other sites show that a diode is placed between two bases of the far right transistors. Mine is a "complimentary-symmetry" amplifier without the diode. It seems that the diode consumes too much power, and the sensitivity is reduced with it in place.

You would be better off replacing the NPN and base resistor with a 50 - 100 ohm resistor.

Wouldn't I lose amplification?

You have no selectivity, so any frequency RF that shows up on the input will be mixed and/or demodulated. Is there a selective RF preamp, or at least a tuned circuit, in front of this?

What I am showing is only the amplifier portion of my receiver. The other part is working for me.

 

[Roff]

Ok, on to the R's and C's. What is the (audio) frequency range of the modulation you are trying to amplify and detect? And what is your speaker impedance?

 

[mstechca]

I use an 8-ohm speaker in all my audio based projects.

What is the (audio) frequency range of the modulation you are trying to amplify and detect?

For now, I want to detect the TV station channel 11. Thats somewhere between 190 and 200Mhz. I want to hear as much as possible without any interference. I'm assuming that would be between 60 and 20000 hz.

 

[audioguru]

Assuming that a 195MHz tuned RF amplifier is feeding this "audio amplifier circuit", it also needs an FM discriminator or ratio detector to detect the FM sound part of a TV station.
I think that the first 3 transistors will be extremely distorted because positive-going input swings for each transistor will be into a very low impedance forward-biased base-emitter junction, and negative-going swings will be into a reverse-biased base-emitter with a high impedance. The resulting high positive current and low negative current in the coupling capacitors will charge them, similar to grid-leak bias. Therefore the transistors will become cutoff during loud audio.
The "complimentary-symmetry" output stage isn't symmetrical having a strong pull-down (if the driver ever gets turned-on) but very weak pull-up. Since the NPN output transistor is operating class-A, the circuit will eat batteries. Needless to say, the circuit doesn't have negative feedback to establish a stable DC operating point nor AC negative feedback to reduce distortion.
I think the whole circuit should be replaced by an LM386 audio small-power IC.

 

[Nigel Goodwin]

Assuming that a 195MHz tuned RF amplifier is feeding this "audio amplifier circuit", it also needs an FM discriminator or ratio detector to detect the FM sound part of a TV station.

It would also need an AM detector at 38MHZ or so, to demodulate the vision carrier which contains the intercarrier FM sound.

There are a great many circuits on the net for audio amplifiers, and many suitable IC's available - the circuit you posting isn't in any way suitable for doing this.