Bidirectional IF amplifier

No, Harry. IF has only modest gain.
Most of TX amplification will take place in TX chain (3 transistors and one Mosfet, and most of RX amplification will take place in low noise AF amplier stage.

I might add a post mixer amplifier after the front end but I'm worried about lowering the dynamic range. The CD4053 if many are connected in parallel as the front end mixer then it will overcome the thermal noise and maybe insertion loss would be 1.5dB only so it will add just 1.5dB of noise so post mixer amplifier will not be necessary. Four CD4053 will have 12 switches. 150ohm per switch / 12 = 12ohm of internal resistance! Sounds good? What do u think? We can use RF amplifier after the ladder filter to add gain and lower the noise figure.

Hi Harry,

The receiver side could have 50dB gain. The IF amp just compensates for Xtal ladder filter loss and losses in the front end RF mixer .

This is another example of a simple 2 transistor bidirectional amplifier. Noise figure is kept low (Only two resistors per transistor). Using a BC549C low noise transistor is suitable here. It's just your basic RF preamps. The diode switching resistors I believe should be lowered to 2.2K so that they pass the required transistor current.. it's an untested idea but it should work. Output impedance is 200ohm to drive a crystal ladder filter. Input Z varies.

Hi again,
I didn't see this page before. This looks very good.

I do have a couple of questions - you are using the same amplifier for TX and RX. If you are using the same IF stage(s) in both directions, then will you need to have a different gain for TX and RX? For TX you are mixing at milli-volts/volts but for receive you are starting with μV.

But I really like that your IF amplifiers have only 10dB gain, so that most of the amplification will be at AF so there is much less chance of IF instability. That always used to be my big problem as I always designed with a huge IF gain + coils: NOT a good combination!! But I learned my lesson 

If you have a low IF gain, then would you be using audio AGC? I recently saw a design for an audio amplifier for an HF receiver that used a "constant-volume" AF amplifier, much like the mic-processor I recently published. The audio fed a speaker via the volume control.

I also saw a design for a a bi-directional IF amplifier that had a different gain, depending on direction, so that the amplifier could be trimmed for TX or RX.

Please DO keep posting the ideas for this project. I for one am really interested.

BR Harry - SM0VPO

Bidirectional amplifier. The input and output Z can be set by changing the value of two 47ohm resistors. This amplifier will properly terminate a crystal ladder filter, etc. Gain is 10dB with a noise figure of 10dB. The 'ON' resistance of a SPDT switch in CD4053 is 150ohm.

The front end mixer is also going to be a CD4053, but double balanced this time. I may use air core coils wound on a 8mm dia. plastic pen.

My goal was to eliminate all hard to find components. CD4053 is widely available both online and in local electronic shops. IF is 4MHz and final mixer VFO would be 3MHz. The CD4053 will work with clock frequencies up to 6MHZ as per the datasheet and nS timings. It will pass signals up to 25MHz .

This 4MHz SSB Exciter is bidirectional. An RF speech clipper made with 3 or 4 x BC547 transistors and an extra crystal ladder filter option would also be there for speech compression. 6dB is common, and it should improve SNR.

The LM386 is used in bridge combination and it produces two AF signals separated by 180 degrees.

Pressing the PTT will generate a short pulse to toggle the four SPDTs for TX/RX and receiver mute function. It's going to be very compact. You guys like the idea?