EMRFD Message Archive 3787

Message Date From Subject
3787 2009-11-19 16:42:24 chrisethompson Micro R2 Op amp stages
I've built the Micro R2 from page 14.75 of the 2009 ARRL handbook but I can't quite get the headphone amplifier to work. It's the same circuit I think as in the Kanga kit.

I've built a seperate audio amplifier circuit and the receiver is now working, but I am left with questions about how the headphone amplifier biasing works.

It looks like the amplifier stages U5A (summer, LPF and amp) and U5B (LPF and amp), are biased from two 27k resistors. However, these resistors are between the I and Q signal paths on the output of the High Pass Filter Op Amps (U1A and U1B). That has me confused. Why are they not from the supply to ground? That is how I understand you bias Op amps that are operated from a single supply.

I started trouble shooting this part of the circuit because when I connect the bias voltage to pin 6 of U5B I suddenly get no audio. Hmmm. That's not supposed to happen I'm sure.

Any thoughts welcome. I think I just don't really understand what the circuit is supposed to do and that is stopping me seeing an error that I have made.

Any suggestions welcome.

3788 2009-11-19 19:27:32 Tayloe Dan-P26412 Re: Micro R2 Op amp stages
It is a bit complex. The output of Q3, if the + supply is about 12v,
will put out ~ 11.5v. R3 and R4 will provide a base voltage of about 2v
to Q1 and Q2. Thus the emitter voltage on Q1 and Q2 will be about 1.5v.
This voltage across R1 and R2 will in turn draw 0.625 mA, which in turn
will cause R6 and R7 to drop 5.7v from the 11.3v out of Q3. That means
U1A and U1B will be biases to 11.3-5.7 or 5.6v.

Whew! This is the real bias voltage for everything. The output for
both U1a and U1b should be about this. This voltage is then being used
as the bias for the other stage via R13 and R14 and C7. U4a and U4b
ought to be at these voltages also. If they are not, it could drive U5a
output to its rail instead of the target of 5.7v.

- Dan, N7VE


3790 2009-11-20 06:18:26 chrisethompson Re: Micro R2 Op amp stages
Wow, thanks for the explanation Dan. I'm going to check the bias voltages tonight and see if I have it setup correctly.

It seems like a really complicated way of getting 5.6v bias, but I suspect there is a really good reason. Why not just use resistors in a divider? I have the same questi
3791 2009-11-20 06:53:07 leon Heller Re: Micro R2 Op amp stages
----- Original Message -----
3792 2009-11-20 07:42:16 Tim Re: Micro R2 Op amp stages
> > Wow, thanks for the explanation Dan. I'm going to check the bias voltages
> > tonight and see if I have it setup correctly.
> >
> > It seems like a really complicated way of getting 5.6v bias, but I suspect
> > there is a really good reason. Why not just use resistors in a divider?
> > I have the same question for the LNA pre-amps. Why not just bias with
> > resistors, rather than through Q3?

Q3 is a "active decoupler" and has existed at least as far back as W7EL's August 1980 QST article. It's important to have good decoupling because in a direct conversion receiver there's so much gain at baseband.

I think the DC coupling all the way through without any interstage coupling capacitors in the Micro R2, is probably a conscious choice made to not introduce any unnecessary components that could mess up the phase shift matching between I and Q channels. I don't think DC coupling is really necessary for phase shift reasons after you get to U5a and U5b where the channels have been combined, but if you've gone that far DC coupled why not do it the last few stages too, rather than introduce a coupling capacitor? As Leon points out the impedances are low so the coupling capacitor would have to be sizable.

Tim N3QE.
3793 2009-11-20 07:48:01 Tayloe Dan-P26412 Re: Micro R2 Op amp stages
Although it seems complicated, it is simpler than using a separate bias
for the transistor pre-amp and the first op-amp gain stage. It is nice
to DC couple audio stages together. Thus, setting up the DC operating
voltages on the pre-amps so that they also provide bias to the first set
of op-amp gain stages, which this set up does, is a good thing..

Taking the output of the first op-amp gain stage and using it for bias
in other places works here because there is no gain between the output
that the bias was taken from and the other inputs it is providing bias
to. The stages between are three unity gain phasing stages and a unity
gain buffer amp driving the low pass filter. When I have tried to use
the same bias for stages early in a gain chain and later in the gain
chain, feedback would tend to happen unless extra measures were taken.
With unity gain from the output point to the input of the other bias
points, that is not really an issue here.

However, in this case you need to make sure the filtering of the bias
from the output (since this output contains detected audio) is
sufficient to make sure that a portion of the first stage output audio
does not effectively bypass the phasing image rejection and low pass
filter stages and undo what you have worked so hard to accomplish. I do
think that a separate voltage divider R/C network for the last two
stages might help further improve performance by eliminating this "blow
by" effect.

That theory could be easily tested by simulating the audio chain using a
freeware simulation package. I like to use SwCAD III from Linear
Technology. I have used it to model the almost the entire audio chain
of my NC2030 from the detector output (modeled as phased voltage sources
with source impedance) to the input to the last SCAF filter. It works
quite well.

- Dan, N7VE


3794 2009-11-20 07:52:08 Tayloe Dan-P26412 Re: Micro R2 Op amp stages
Good point about the phasing aspect at low frequencies where the AC
coupling could have an impact. I guess that I have not noticed that
point as much in my designs as I have tried to add high pass filtering
after the phased image rejection point to try to get away potential 60
and 120 Hz hum.

- Dan, N7VE


3795 2009-11-21 08:16:41 chrisethompson Re: Micro R2 Op amp stages
Thanks for everyones advice. I understand this now and everything is working. It turns out that there is an error in the schematic in the ARRL handbook. U5B has the inverting and non-inverting inputs labeled back to front. When I applied the bias and the input correctly, everything worked as expected.

An initial tune around the band was amazing. When you are used to a direct conversi