AGC-controlled Termination Insensitive Amplifier experiment

The Termination Insensitive Amplifier (TIA) is a popular wide-band RF and IF amplifier, and a foundation of the Bitx, uBitx and many other superhet radios. A minor draw-back is its fixed gain.

In this video, I experiment with a mechanism (proposed by George VK4AMG) to add AGC to a TIA IF amplifier stage. The mechanism varies emitter degeneration and is similar to a scheme using PIN diodes shown in EMRFD Fig 6.41 (which is not a TIA).

This mechanism works, but there are side effects. Is it a useful hack? Tell me what you think!

Thanks to George VK4AMG, Bill N2CQR and Peter N6QW for helpful comments and feedback.

'A Termination Insensitive Amplifier for Bidirectional Transceivers', Wes Hayward, w7zoi, and Bob Kopski, k3nhi. © 26 June 09:

Facebook (VK Homebrew), scroll down to George VK4AMG's post, March 3rd 2021:

Blog post:

Disclaimer: I am not an RF design engineer and I offer this as an interesting experiment to home builders and experimenters. If you want to challenge or debate the approach, I encourage you to model, build, measure and report your findings!

[vkaqz]

Thanks for the video Paul. Nice to see someone trying out circuits. I have used similar circuits for gain control both at RF and audio. You can get good results by also using a FET in the same circuit. Hayward actually shows the use of 1 and 2 PIN diodes but they introduce distortion at low RF frequencies due to short charge time in the intrinsic layer (Ref Hayward, RF design page 255). The simplified gain equation for an un bypassed emitter is the collector resistor (or impedance) divided by the emitter resistor. So with a 220R collector resistor, and a 100R emitter resistor the voltage gain is 2. The transistor or FET or diode may put, say, a 10R equivalent resistor across the 100R which then gives a voltage gain of 22 times, which is around 27 dB. And going down to 5 ohms, gives a gain of 33 dB etc. However I found that as you increase the gain, the third order IMD gets worse and at a very low RF emitter impedance, the transistor starts to go from a linear class A RF amplifier to a less linear mode. Fiddling with the emitter impedance also lowers the base input Z. That is, it goes from a lot of negative emitter feedback with a high input Z (as in an emitter follower), to a lot less negative feedback, hence gain more gain, but lower input Z. So the amplifier input impedance goes up and down with AGC not a good situation if fed from tuned circuits or crystal filters. The best AGC controlled system is the AD603 IC which does not use variable degeneration but an internal voltage controlled resistor network resulting in low IMD at all values of AGC and with high dynamic range.

[MNTVHomebrewing]

Hi Paul. Thanks for another fascinating video. We've corresponded about this before as I used fixed gain TIAs in my recent Optimiser rig. As I actually daisy chain a couple of these modules on either side of my crystal filter I did wonder about putting some variable IF gain in-between each pair of TIAs - like a kind of AGC sandwich! 

i.e. TIA -> Variable Gain IF Amp -> TIA -> Crystal Filter -> TIA -> Variable Gain IF Amp -> TIA

The TIAs would have to be built for pretty low gain so as not to overdrive. My thinking was that the TIAs would effectively buffer the connecting stages from any impedance changes whilst still allowing me some amount of AGC control before and after the filter. I haven't done any more than think about it yet but its on the TODO list! Thanks again. Great stuff! 73 Nick M0NTV

[yakovdavidovich]

If you preceded that with a modest gain low noise amplifier (e.g., grounded gate JFET or something that won't compromise dynamic range), you could then follow that with a pad to stabilize the impedance mismatch. E.g., [LNA]->[PAD]->[TIA]. I've been playing with circuits based on EMRFD 6.32 and following for a direct conversion receiver, and your comments at the end reminded me of it. Probably crazy, but it was my first thought :-).

[RAUK]

Very interesting. This approach is similar to EMRFD Fig 6.41, but instead of a NPN transistor authors choose to use a PIN diode as a voltage variable resistor (1N4148's work as well for low-level signals). The amplifier described there is not TIA though. I was thinking about building an IF derived AGC with TIA as well, and came up with another approach. Instead of changing the gain, it's possible to place a voltage variable attenuator on IF stage. I discovered that the circuit proposed by Raymond Waugh (Google for Waugh attenuator, it's easy to find) works well with 1N4148's, is well matched on HF (and up to ~150 Mhz if you don't mind SWR <= 2.5), and is bidirectional. I'm going to try it in my next transceiver.

[WhatDennisDoes]

The fact of the matter is that it worked, and we heard the result. So then what is the concern? Is impedance matching on receiver blocks really all that critical anyway? I don't know the answer, I've only scratch built one rig so far. But for the sake of argument, say you have an antenna preamp going into a diode mixer then this IF strip. The mixer in this case isn't changing the output impedance of the stage before it right? So a varying input impedance would look like a varying output load on the preamplifier, which would only mean the gain is changing. But you are also varying the gain of the IF amp to compensate... Seems it might be a wash. Does this make any sense? I'm kind of thinking out loud. :)

[paulsradiohacks]

Ive used this circuit or a very similiar one as the first RF amp before the first mixer in my current design, Im finding the gain drops off as the frequency increases.... so at say 10mhz its a healthy 20dB but drops off fast at 28MHz to around 10dB any ideas why this would happen? Im thinking capacitance in the inductor L