Superheterodyne Receiver Book

Thread Starter

KCHARROIS

Joined Jun 29, 2012
302
Ok so what was I calculating...

And now how do I relate this to selectings my secondary winding to couple the stage to another?
 

Audioguru

Joined Dec 20, 2007
11,249
I don't know what you were calculating. It was not the input impedance of the transistor.

You must detemine how much load reduces the Q of the tuned circuit so the Q is too low.
 

vk6zgo

Joined Jul 21, 2012
677
Back in the day, IF transformers were commercially available,as were antenna coils, RF stage coils & oscillator coils,so making a Superhet was considerably simpler.

In tube times,RF & IF coils had both primary & secondary windings as tuned circuits,with similar impedance,as tubes are a high impedance device.

With the advent of BJTs,the secondary needed to be of a lower impedance to the primary.

This could be done several ways:

(1) A tuned secondary with the transistor base tapped down on the winding.
(2) An untuned secondary like the circuit you have provided.

(1)Has an additional problem,that three element devices like triode tubes & transistors are prone to oscillation if they have tuned circuits in both the input & output,& need a special form of negative feedback,known as neutralisation to obtain stable operation.

Option (2) avoids this problem,at the expense of some selectivity & stage gain.
 

Thread Starter

KCHARROIS

Joined Jun 29, 2012
302
So I found a website showing how to design tuned rf amplifier. So heres a picture of a circuit. Is it safe to assuma that RL is equal to (12V-2.31V)/25mA ≈ 388Ω

Thanks

 

Audioguru

Joined Dec 20, 2007
11,249
Is it safe to assuma that RL is equal to (12V-2.31V)/25mA ≈ 388Ω
No.
You calculated the average resistance of the transistor, not the much higher impedance of the parallel-tuned LC circuit.

The input impedance of this transistor circuit is the internal emitter resistance of about 1 ohm in series with the 56 ohm unbypassed emitter resistor times the beta of the transistor (100?) in parallel with the base voltage divider resistors. Maybe about 551 ohms.
 

Thread Starter

KCHARROIS

Joined Jun 29, 2012
302
Ok I understand that but are hes calculations correct? I can modify the circuit to fit my needs. You calculated RL differently then him and I dont quite understand does his RL have a different meaning then your RL?

Thanks
 

Thread Starter

KCHARROIS

Joined Jun 29, 2012
302
I understand that when looking at DC measurements that the resonant circuit looks like nothing to DC voltage. The cap blocks DC and the inductor looks like a wire. There for the gain can only be by calculating the average resistance of the transistor which is ≈ 388Ω. There for my gain is ≈ 7 because 388/56 = 6.92 not including r'e = 26/25 = 1.04.
 

Audioguru

Joined Dec 20, 2007
11,249
I understand that when looking at DC measurements that the resonant circuit looks like nothing to DC voltage. The cap blocks DC and the inductor looks like a wire. There for the gain can only be by calculating the average resistance of the transistor which is ≈ 388Ω. There for my gain is ≈ 7 because 388/56 = 6.92 not including r'e = 26/25 = 1.04.
No.
The RF gain is very high because the tuned parallel LC circuit is a very high impedance when it is not loaded down. The inductance cancels the capacitance at resonance.
The voltage gain of a transistor is the collector load impedance divided by the unbypassed emitter resistances.
 

vk6zgo

Joined Jul 21, 2012
677
You don't really need several stages of RF amplification prior to the mixer,in fact many radios don't have any,& simply have a tuned circuit at the mixer input.
The whole idea of a Superheterodyne is to not have to have the hassles of tuning multiple circuits as you go.

I just had a look at my Jaycar catalogue,& they have under "radio coils".a set of Oscillator coil & IF transformers for a MF AM radio with a 455kHz IF.

As I sorely doubt these things are only made for that one company,why not see if you can buy two sets in your country.
Use one to build the radio,& one set to dismantle & count the turns,or whatever you think is good.

You are continually drifting off your original path--you don't have to know how to design every little bit of the thing.

Just build the radio,then later on,revisit all this other stuff,perhaps with the additional insight of having played with a real radio.
After all,when you build a house,you don't study how to make the bricks & tiles!
Or do you?:D
 
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