Sorry about that John, I checked the link in an incognito window so it should work without a login.

Phil the XOR gate was just an experiment to try and get to a working design and iterate from there. I have now got everything working again with a two-transistor NAND gate. This is of course the same as an XOR gate only it goes from VCC/2->VCC instead of 0->VCC. Now I don't have any problems with trying to make an amplifier where the input goes to the negative rail :D. (I couldn't get that to work anyway, even with pen and paper).
This also means that a simple current mirror can be used to bias the diff amp that previously required a negative rail. The output of course no longer goes to the negative rail either, but a couple of diode connected transistors takes care of that!
https://photos.app.goo.gl/b3rB4Nvdydrg7cAo6

Overall the design isn't particularly elegant but it does work whilst sticking to my original principles, and that's enough for me at this point. I just want to finish the damn thing.

On 7/22/2022 9:55 PM, Harry Dudley-Bestow wrote:
> Well time has gone by and I have got the PLL working!
> Input to a VCO is green, output from the PLL control signal is blue:
> https://photos.app.goo.gl/WtgUkvxBuhWBNv9k6


It wants to verify i'm me. Just to view a photo. So screw it!


> It locks from between about 10-11MHz and can be used to demodulate FM audio just fine.
> For the final "productionised" version I would like to remove the negative rail I am using for my loop filter amplifier.
> Currently the loop filter is a simple two transistor diff amp:
> https://photos.app.goo.gl/7hiqrjRzVVpj3TqC6
> However in order for the input to go to 0V, this requires a negative voltages source which for a battery powered device is not ideal.
> 
> My question is: What is the simplest possible amplifier where the input can go to the negative rail?
> 
>  From some reading in the "Design of Analog CMOS Integrated Circuits" I understand that something may be possible with a pmos diff pair, but I cannot get anything to work in simulation. @Phil Hobbs any ideas?

Harry Dudley-Bestow wrote:
> On Sunday, July 24, 2022 at 12:10:41 PM UTC-7, Phil Hobbs wrote:
>> Harry Dudley-Bestow wrote:
>>> Thanks for the input.
>>> 
>>> Hul I am aware of rail to rail input amps :). What I would like
>>> to do for my project is make something using discrete
>>> components.
>>> 
>>> I will try out the PNP diff pair and see how it goes. I am not so
>>> sure where this "resistive pulldown is supposed to go but all
>>> will become clear, I'm sure.
>>> 
>> Two PNPs connected emitter to emitter, with the bases connected
>> across whatever phase detector you're using. (It'll need to be
>> biased above ground if you want any significant output swing from
>> the amp.)
>> 
>> Resistor from the emitters to the supply to provide bias current.
>> 
>> One collector grounded directly, the other connected to ground via
>> a resistor that provides the output voltage to the varactor.
> 
> Thanks for the clarification Phil. That's more or less what I had
> going in LT spice. The tip about biasing the input higher though got
> the circuit to work in the sense that it provides some gain, but
> that's not sure what I want. here's what I have now: 
> https://photos.app.goo.gl/Zp2wmpdBcCtxxsUU7 For reference here is the
> circuit I'm using now that works just fine, the only downside being
> the requirement for a negative rail: 
> https://photos.app.goo.gl/FCmdL4UFaEXsSFUV7
> 
> I'm using an XOR phase detector (The double balanced mixer gave me no
> end of trouble) and so the output of that after filtering is 0->2.5V
> and experimentally I've determined a gain of about 5 is what I want. 
> Correct me if I'm wrong here but the problem is that the output of
> that circuit can't be zero because of the quiescent current through
> the resistor on the collector of the pnp (Rc2 in my schematic). I can
> bias the input voltage to be higher with a simple pullup on the base
> of the input pnp, but I really need the output to be 0V when the
> input is 0V, else my varactor will have too high of a voltage on it
> to begin with. The output range also doesn't seem that crash hot
> (1.1->2.2V) either but I presume that I can fix that by fiddling with
> the current source and the output collector resistor.

If you're OK using an XOR gate, just use an op amp and move on.

Alternatively, gain up the 5V swing, and use that directly with an RC 
lead-lag filter.  (For FM demodulation you don't care about reference 
frequency ripple, after all.)

Otherwise, putting down the mouse and doing a little bit of algebra will 
make things clear in a big hurry.  Those sorts of questions are easy to 
figure out with a bit of thought.

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On Sunday, July 24, 2022 at 12:10:41 PM UTC-7, Phil Hobbs wrote:
> Harry Dudley-Bestow wrote: 
> > Thanks for the input. 
> > 
> > Hul I am aware of rail to rail input amps :). What I would like to do for my project is make something using discrete components. 
> > 
> > I will try out the PNP diff pair and see how it goes. I am not so sure where this "resistive pulldown is supposed to go but all will become clear, I'm sure. 
> >
> Two PNPs connected emitter to emitter, with the bases connected across 
> whatever phase detector you're using. (It'll need to be biased above 
> ground if you want any significant output swing from the amp.) 
> 
> Resistor from the emitters to the supply to provide bias current. 
> 
> One collector grounded directly, the other connected to ground via a 
> resistor that provides the output voltage to the varactor. 

Thanks for the clarification Phil. That's more or less what I had going in LT spice. The tip about biasing the input higher though got the circuit to work in the sense that it provides some gain, but that's not sure what I want. here's what I have now:
https://photos.app.goo.gl/Zp2wmpdBcCtxxsUU7
For reference here is the circuit I'm using now that works just fine, the only downside being the requirement for a negative rail:
https://photos.app.goo.gl/FCmdL4UFaEXsSFUV7

I'm using an XOR phase detector (The double balanced mixer gave me no end of trouble) and so the output of that after filtering is 0->2.5V and experimentally I've determined a gain of about 5 is what I want.
Correct me if I'm wrong here but the problem is that the output of that circuit can't be zero because of the quiescent current through the resistor on the collector of the pnp (Rc2 in my schematic). I can bias the input voltage to be higher with a simple pullup on the base of the input pnp, but I really need the output to be 0V when the input is 0V, else my varactor will have too high of a voltage on it to begin with.
The output range also doesn't seem that crash hot (1.1->2.2V) either but I presume that I can fix that by fiddling with the current source and the output collector resistor.

> Some appropriate bypass capacitor across the resistor to give a generous 
> PLL bandwidth without letting much of the RF get lost in the resistor.
> Cheers 
> 
> Phil Hobbs 
> 
> -- 
> Dr Philip C D Hobbs 
> Principal Consultant 
> ElectroOptical Innovations LLC / Hobbs ElectroOptics 
> Optics, Electro-optics, Photonics, Analog Electronics 
> Briarcliff Manor NY 10510 
> 
> http://electrooptical.net 
> http://hobbs-eo.com

Harry Dudley-Bestow wrote:
> Thanks for the input.
> 
> Hul I am aware of rail to rail input amps :). What I would like to do for my project is make something using discrete components.
> 
> I will try out the PNP diff pair and see how it goes. I am not so sure where this "resistive pulldown is supposed to go but all will become clear, I'm sure.
> 

Two PNPs connected emitter to emitter, with the bases connected across 
whatever phase detector you're using.  (It'll need to be biased above 
ground if you want any significant output swing from the amp.)

Resistor from the emitters to the supply to provide bias current.

One collector grounded directly, the other connected to ground via a 
resistor that provides the output voltage to the varactor.

Some appropriate bypass capacitor across the resistor to give a generous 
PLL bandwidth without letting much of the RF get lost in the resistor.

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

Thanks for the input.

Hul I am aware of rail to rail input amps :). What I would like to do for my project is make something using discrete components.

I will try out the PNP diff pair and see how it goes. I am not so sure where this "resistive pulldown is supposed to go but all will become clear, I'm sure.

Harry - there are numerous op amps that allow the input below the negative rail. I can't recall a specific number though. The rail to 
rail devices Phil mentions might be a good starting point for a search. These have, I believe, both p & n channel devices on the input. 
The ones I vaguely rember were input from below negative to within 2 volts of positive.

Hul


Harry Dudley-Bestow <harry.dudleybestow@gmail.com> wrote:
> Well time has gone by and I have got the PLL working!
> Input to a VCO is green, output from the PLL control signal is blue:
> https://photos.app.goo.gl/WtgUkvxBuhWBNv9k6

> It locks from between about 10-11MHz and can be used to demodulate FM audio just fine.
> For the final "productionised" version I would like to remove the negative rail I am using for my loop filter amplifier.
> Currently the loop filter is a simple two transistor diff amp:
> https://photos.app.goo.gl/7hiqrjRzVVpj3TqC6
> However in order for the input to go to 0V, this requires a negative voltages source which for a battery powered device is not ideal.

> My question is: What is the simplest possible amplifier where the input can go to the negative rail?

> From some reading in the "Design of Analog CMOS Integrated Circuits" I understand that something may be possible with a pmos diff pair, but I cannot get anything to work in simulation. @Phil Hobbs any ideas?

Harry Dudley-Bestow wrote:
> Well time has gone by and I have got the PLL working! Input to a VCO
> is green, output from the PLL control signal is blue: 
> https://photos.app.goo.gl/WtgUkvxBuhWBNv9k6

Nice.

> 
> It locks from between about 10-11MHz and can be used to demodulate FM
> audio just fine. For the final "productionised" version I would like
> to remove the negative rail I am using for my loop filter amplifier. 
> Currently the loop filter is a simple two transistor diff amp: 
> https://photos.app.goo.gl/7hiqrjRzVVpj3TqC6 However in order for the
> input to go to 0V, this requires a negative voltages source which for
> a battery powered device is not ideal.
> 
> My question is: What is the simplest possible amplifier where the
> input can go to the negative rail?
> 
> From some reading in the "Design of Analog CMOS Integrated Circuits"
> I understand that something may be possible with a pmos diff pair,
> but I cannot get anything to work in simulation. @Phil Hobbs any
> ideas?
> 

Well, there are lots of RRIO CMOS op amps out there, e.g. .  If you want 
something discrete, a PNP diff pair with a resisitive pulldown would 
work, and be a lot quieter than discrete small-signal PFETs, which are 
pretty scarce nowadays anyway.

Something like that obviously won't pull down very strongly near ground, 
but then all you're driving is a varactor, right?

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

Well time has gone by and I have got the PLL working!
Input to a VCO is green, output from the PLL control signal is blue:
https://photos.app.goo.gl/WtgUkvxBuhWBNv9k6

It locks from between about 10-11MHz and can be used to demodulate FM audio just fine.
For the final "productionised" version I would like to remove the negative rail I am using for my loop filter amplifier.
Currently the loop filter is a simple two transistor diff amp:
https://photos.app.goo.gl/7hiqrjRzVVpj3TqC6
However in order for the input to go to 0V, this requires a negative voltages source which for a battery powered device is not ideal.

My question is: What is the simplest possible amplifier where the input can go to the negative rail?

From some reading in the "Design of Analog CMOS Integrated Circuits" I understand that something may be possible with a pmos diff pair, but I cannot get anything to work in simulation. @Phil Hobbs any ideas?

> Not necessarily. You can put an FM signal through a frequency 
> multiplier, for instance, and get perfectly good sound nonetheless. 
> 
> Thing is, the waveform is an amplitude thing, whereas the modulation is 
> a phase thing. 
> 
> Cheers
> Phil Hobbs 
> 
> 
> -- 
> Dr Philip C D Hobbs 
> Principal Consultant 
> ElectroOptical Innovations LLC / Hobbs ElectroOptics 
> Optics, Electro-optics, Photonics, Analog Electronics 
> Briarcliff Manor NY 10510 
> 
> http://electrooptical.net 
> http://hobbs-eo.com

Interesting! In that case, I shall proceed onwards with constructing the receiver, and deal with new problems as they arise.