Intermediate Frequency Amplifier.

[Chiranka K]

Hey, i was designing a discrete FM radio receiver using transistors only (no dedicated IC's). I am now at a stage of designing the IF stage. I wanted help with these few topics

1. A tuned IF filter. I did google for resources, i found a few books which did give me reliable circuits. However they seemed like a theoretical circuits.

I wanted help with some practical circuits that the fellow members of the forumn have already tried.
The circuits i say used transformers at input and output, why are transformers and coupled inductors used to such extent in RF circuits?

Thank you

 

[FvM]

Here's a classical FM IF amplifier with detector, copied from Siemens circuit examples 1970/71. Core component is tunable coil assembly. Each filter is implemented with two coupled LC resonator, a low impedance output winding matched to base terminal of next stage and a neutralisation winding as special feature.

--- Updated Dec 16, 2024 ---

 

[danadakk]

Google this "amateur radio build if transformer"

 

[vfone]

As was mentioned, the schematic on #2 use neutrodyne circuits. Neutrodyne was invented 100 years ago to prevent instability in RF tubes by cancelling out (or neutralized) the parasitic oscillations.
In tubes amplifiers neutrodyne is a bit easier to be implemented due to lower gain of the active devices, but is very hard to be implemented in solid-state devices, which have much higher gain. For a beginner in RF is practical impossible to start designing RF circuits with one like this.

 

[Chiranka K]

As was mentioned, the schematic on #2 use neutrodyne circuits. Neutrodyne was invented 100 years ago to prevent instability in RF tubes by cancelling out (or neutralized) the parasitic oscillations.
In tubes amplifiers neutrodyne is a bit easier to be implemented due to lower gain of the active devices, but is very hard to be implemented in solid-state devices, which have much higher gain. For a beginner in RF is practical impossible to start designing RF circuits with one like this.

What kind of circuits do u think i can build.

--- Updated Dec 17, 2024 ---

This was a general tuned amplifier that i could find, with L=10uH, and C=22pF. The respsonse however is kinda not how its supposed to be.

Google this "amateur radio build if transformer"

 

[FvM]

The respsonse however is kinda not how its supposed to be.

R1 creates unwanted wide band transmission. If you want R1 for some reason, interchange resonator and R1 and connect output at resonator-R1 node. Or use separate output winding.

 

[Chiranka K]

I designed a staggered IF amplifier with centre freqeuncy around 10Mhz, the BODE of the amplifier looks good.

But the output for a sine wave input of (10Mhz, 5mV looks like this)

I dont think so its a problem with transistor, because ive used this transistor to build a oscillator of 100MHz. What do u think the problem is

--- Updated Dec 17, 2024 ---

R1 creates unwanted wide band transmission. If you want R1 for some reason, interchange resonator and R1 and connect output at resonator-R1 node. Or use separate output winding.

it was just a circuit i found online, didnt really put any thinking to it. However in #7, i did a bit more research and chose this confuguration, its more like a cascaded amplifiers, the bode was good, but the output looks saturated.

The LC tank in itself arent giving proper sine waves, but saturated waves, the input at the gate of the first transistor also, isnt just (Quiescent base voltage + Small signal voltage).

Im not able to debug this error

 

[FvM]

The LC tank in itself arent giving proper sine waves, but saturated waves, the input at the gate of the first transistor also, isnt just (Quiescent base voltage + Small signal voltage).

Im not able to debug this error

Not necessarily an error. FM IF amplifiers are mostly designed without gain control (AGC), saturation is expectable. But you need provisions that saturation doesn't kill resonance. Collector series resistors in post #2 schematic have this purpose.

 

[Chiranka K]

Not necessarily an error. FM IF amplifiers are mostly designed without gain control (AGC), saturation is expectable. But you need provisions that saturation doesn't kill resonance. Collector series resistors in post #2 schematic have this purpose.

this makes sense, any resources like textbooks/lectures where i can learn in depth about this?

Not necessarily an error. FM IF amplifiers are mostly designed without gain control (AGC), saturation is expectable. But you need provisions that saturation doesn't kill resonance. Collector series resistors in post #2 schematic have this purpose.

What dB of Gain is expected from the amplifier usualyy?

--- Updated Dec 17, 2024 ---

Not necessarily an error. FM IF amplifiers are mostly designed without gain control (AGC), saturation is expectable. But you need provisions that saturation doesn't kill resonance. Collector series resistors in post #2 schematic have this purpose.

Yes, it seems to work now. Thanks a lot for ur input.
Getting a gain of about 9dB, will try to improve upon it with further design iteration

 

[vfone]

If you want a narrow band, high gain (40dB) and stable amplifier, you have to follow the rules presented in the schematic attached.
First of all, to get better amplifier stability is to add the loading resistors in emitter instead of collector. They do the same thing, but placed in emitter they affect less the RF gain and the bandwidth of the amplifier.
An option to get better stability for a two stage amplifier is to use a capacitor divider between stages (C3 and C4 in my schematic). This will improve stability and also provides impedance matching between stages.
The same divider is used at the output (C6 and C7) to provide output impedance matching.
I don't think you need such higher gain (40dB) but this example is good to understand how things goes.

 

[Chiranka K]

If you want a narrow band, high gain (40dB) and stable amplifier, you have to follow the rules presented in the schematic attached.
First of all, to get better amplifier stability is to add the loading resistors in emitter instead of collector. They do the same thing, but placed in emitter they affect less the RF gain and the bandwidth of the amplifier.
An option to get better stability for a two stage amplifier is to use a capacitor divider between stages (C3 and C4 in my schematic). This will improve stability and also provides impedance matching between stages.
The same divider is used at the output (C6 and C7) to provide output impedance matching.
I don't think you need such higher gain (40dB) but this example is good to understand how things goes.

wow! This circuit seems to solve all the problems.
1. Do u mind telling how did u arrive at this circuit and this biasing. Which resource did u use?
2. How is the tuning happening here? which capacitors and inductors play a part?

Thanks a lot