EMRFD Message Archive 14562

Message Date From Subject
14562 2018-03-02 11:01:26 Ashhar Farhan signal handling of audio-preamp in receivers
i have been brewing an new receiver audio system. this uses the standard common-base preamplifier with a directly coupled pnp amplifier in common emitter mode. i guess this has been a staple of many a receivers - from kk7b's microR1 to Easy-90 of Fig 6.72 in the EMRFD.

For a direct conversion receiver like the microR1 or microR2, there is no AGC before the audio pre-amp. Hence, one expects these audio preamps to be strong enough to not distort loud signals. The one that I reproduced from the progressive receiver. It shows a voltage gain of 30 db. Thus, a 50uv signal produces an output of 50 mv. So far, so good. 

Now, as I increased the audio, at whereabouts of 500mv output (from a 500uv input) the audio started to distort badly. The harmonic distortion crept up to almost 1%. Isn't this a challenge? 

I do understand that there should be attenuation in the front-end, but given that we routinely design RF stages to handle 80 db or better, it is a bummer that the audio preamp shows compromise. 

Any thoughts?

- f  
14563 2018-03-02 11:14:09 Tayloe, Dan (Noki... Re: signal handling of audio-preamp in receivers

It is more expensive, but rail-to-rail output, low noise op-amps do well in this spot to help handle a higher dynamic range without the use of AGC. For a DC receiver, the detected bandwidth can be quite high, with large off-channel signals.  Thus for higher dynamic range, it is better to use a rail-to-rail output op-amp followed by good low pass filtering before the final amplification.


I also like R/C active low pass filters because they don’t introduce loss and magnetic pickup (AC hum, etc.), and because the filter can introduce gain if the gain per stage is kept low (like 2x or less per stage).


It is a cost/complexity/performance tradeoff.


  • Dan, N7VE


14564 2018-03-02 13:06:15 tim Re: signal handling of audio-preamp in receivers
It's not just a matter of dynamic range. It's also a matter of the
operating curve of the amp.

You can get an 80db rf gain in a small-signal rf stage using a 9v
supply. The signals do not drive the stage anywhere near non-linearity.

In an audio stage, however, it's easy to drive the stage past the
linear operating point because of the voltage levels involved. If you
have a 9v supply you can easily drive the stage into saturation. You
only need a stage gain of about 15 to 20 with a 500mv input to totally
saturate the stage and cause lots of distortion.

You may need to run your audio stage at a higher voltage or use a
+9v/-9v power supply.

tim ab0wr

14565 2018-03-02 14:32:04 Tayloe, Dan (Noki... Re: signal handling of audio-preamp in receivers

That was not the point.  The problem quoted was a 30 dB gain with a 0.5v output saturation,  Using a low noise device like a LT6231 running at 12v, you can get close to a rail-to-rail output.  Using the same 30 dB pre-amp gain going into a phasing/filtering section, you can get over 20x higher output for a 16 dB larger dynamic range improvement.


That said, the NC2030 (a phasing DC transceiver) got 140 dB of blocking dynamic range (20 KHz out) out of a 3v receiver using op-amps and 12 ma of total receiver current drain.  It did help that the quadrature detector has a built in single point R/C roll off (system input resistance is the series R, the detection caps are parallel C forming an audio low pass filter), which does help reduce the amplitude of large out of channel signals.


The 3v 4:1 analog multiple chip in the detector (like a FST3257) does help make sure that the mixer is capable of handling the signal.  If the output was 10v pk-pk, using a 30 dB preamplifier, the input could be as larger as 0.31v pk-pk (-16 dBm across 50 ohm) before saturating the  pre-amp, which is a pretty hefty signal.


It is always a tradeoff to reduce the gain and increase the input high level signal handling vs. lowering sensitivity.  A lot of that depends on the noise level contribution of the op-amp after preamplification vs. the noise level contribution of the following stages.


All that said, I am not sure that it matters.  Unless your goal is to produce a very high performance DC receiver, 0.5v pk-pk out of the pre-amplifier is probably good enough as most DC receivers are pretty simple with relatively low performance.  The NC2030 receiver was very stout and had high sensitivity, but the heavy R/C active filtering in the receiver chain used a lot of parts.


  • Dan, N7VE


14566 2018-03-02 18:49:36 tim Re: signal handling of audio-preamp in receivers

Ashar was talking about the audio chain. I'm not sure the calculations
he is using are correct. A voltage gain from 50uv to 50mv is gives a
voltage ratio of 1000. This is a voltage gain of 60db, not 30db. A
voltage gain of 60db from a direct coupled pnp common-emitter amplifier
is pretty darn good. Usually you would expect a voltage gain of about 10
to the first approximation with a collector load of 1k and an emitter
resistor of 100. If the emitter resistor is completely bypassed you
could see a gain as high as 200 (limited by the internal resistance of
the transistor).

If Ashar is really seeing a voltage gain of 1000 out of a
common-emitter amp I'm not surprised he's seeing distortion. My guess
the amp is no longer operating on a linear load line in Class A in this

tim ab0wr

14567 2018-03-02 19:20:59 Tayloe, Dan (Noki... Re: signal handling of audio-preamp in receivers
I know that I also have looked into the pre-amp circuit that is used out of the mixer of an R2 type DC receiver.  It has a good 50 ohm input, it has low noise, it is cheap and it provides about 30 dB of gain. I thought this is what he was referring to.

However, when I modeled it, it did appear that it had a limited output swing.  Thus I am a bit familiar with the issue.  It works great, but does have a somewhat limited output swing. A good op-amp is a lot more expensive, but it has a much better output swing and likely lower distortion.

- Dan, N7VE

Sent from my T-Mobile 4G LTE device

------ Original message---
14568 2018-03-03 06:50:13 tim Re: signal handling of audio-preamp in receivers
I agree an op-amp would be a much better choice. Trying to get large
amounts of gain out of a common-emitter amp requires such a large
collector resistor that the amp becomes basically a switch and not an
amp. Very small changes in base current quickly drives Vce from max to

A voltage gain of 30db is a stage gain of about 50. That's doable with
a bypassed emitter resistor and something like a 1200ohm:8ohm audio

tim ab0wr

14569 2018-03-03 11:03:33 Ashhar Farhan Re: signal handling of audio-preamp in receivers
i had clearly saud that it is a voltage gain of 30db. You must also realize that the output imepdance is different from the input impedance. the input impedance is approximately 50 ohms and the output impedance is 3000 ohms. if we 'normalized' the output impedance to 50 ohms thorugh a hypothetical transformer of 7:1 winding, then the voltage gain would be  about 130 (21 db). The power gain would be 40 db. the version that i built was from the progressive receiver. i thought i was doing something wrong until i worked the circuit out on a piece of paper, then it hit me.

I think it is the collector load resistor that is responsible for flat-topping. If that were replced with a tranformer, the situation would be much improved. 
The other option is, to of course do what Dan suggests, drop in an op-amp. As the noise figure is already established by the an common-base amplifier, the next stage can be a low-cost NE5532 as well. I must try this configuration out and report results.

I expect, even from a less challenging location such as mine, that AGC-less receivers will routinely have to handle signals in the 80 db range. We are talking of signals from 1uv to 10mv. The circuitry before the manual gain control should be able to handle this without distorting. Louder signals should be just that 'loud' not distorted. we will have to consider this in the simpler designs. it turns out that the AGC actually helps the more compromized superhets perform better by shielding their later stages from high level signals and their in-channel distortion.

btw, i took vasily's advice on audio amplifiers to heart. He recommended a low distortion sinewave generator and measuring harmonic distortion with it while evaluating audio chains. It was a breeze to do that without really much of  effort. My laptop has 24-bit audio. A very low distortion sinewave from the sound output looped back into it's sound input showed -60 dbc harmonic distortion. the second order product climbs up almost linearly with power. 

- f

14582 2018-03-11 10:39:34 jim_amos_n8cah Re: signal handling of audio-preamp in receivers

I have (I think) the same audio preamp on my workbench as part of a prototype receiver.    One thing that struck me though was that the original application (including mine) was for a superhet RX running AGC, and not a DC receiver without AGC.    So, the expected input signal range for the original application are different as the AGC will control the level of the input signal. 

The output swing in the discussed circuits is being limited by how the transistors are being biased.   In a classic CE amplifier, the collector load resistor is placed at 1/2 VCC allowing for the output to swing above and below that midpoint. The transistor being discussed has an additional 3.3K in series with the emitter that is there to help set the bias by dropping supply voltage.   This resistor is dropping 3.3V at a collector current of  1mA (measure this, it might be more), so this is 3.3V that is not available as part of your output sing.   From a dB perspective this is pretty minor though. 

You might try turning this problem around though.   Think about how much power / output swing you need at that point, and design around that.   You would then set your output bias / gain to meet this requirement.    

One option would be to lower the gain of these stages instead of beefing up the output for more drive.   This can be done by increasing the 220 ohm in series with the emitter since it's being used for emitter degeneration.   This might make more since if you are following with some additional filters, etc.    In my application I'm driving a volume pot into an LM380 amplifier and seems to functi