Let's talk about the Jordan Bosstone. An odd one...

[Rob Strand]

Seems hard to imagine that the designer came up with this on a piece of paper, thinking it was an obvious solution and then no-one ever again though the same way.

How you come-up with these things depends on your mind set.

I see it as a bootstrap technique - very common way of getting more gain.
Groovenut's post is an equally valid way to start the idea, as a second step you then add RB to fix the bias current.

[PRR]

> I wonder how this thing was first designed.

Early transistor writings listed zoo-fuls of possible connections. From Cowles:



Here is how I see the AC equivalent. "Practical" details like batteries, bias, ground omitted.

[Rob Strand]

Early transistor writings listed zoo-fuls of possible connections. From Cowles:

Interestingly if you take fig 10.3 and change the second stage from a PNP CE  stage to an NPN CE stage you get about 20dB more gain (because it forces Q1 to operate at a lower collector voltage).   That pattern is essentially the Fuzz Face pattern (although Fuzz Face has a lot more biasing baggage.)

[PRR]

> if you take fig 10.3 ...... essentially the Fuzz Face ....has a lot more biasing baggage.)

That collage omitted biasing.

Consider fig 10.4:

[Rob Strand]

That collage omitted biasing.

Consider fig 10.4:

Thanks, very cool.

I remembered I posted this some time ago,
http://www.diystompboxes.com/smfforum/index.php?topic=110917.0

[jatalahd]

The Bosstone circuit presented at the start of this thread implements a typical shunt-shunt global feedback configuration over Q1 and Q2, where the feedback network (560k + 0.022uF + 560k) moves from closed to open loop along with growing frequency through C2. The schematic below shows my simplified analysis version, where the simulation model is on the left side and the small-signal model on the right side:



From the basic feedback theory, the gain for the shown circuit in closed loop (low frequency limit) is (RB1a+RB1b)/RS = (560k+560k)/10k => 40 dB. In open loop (high frequency limit) the total gain is determined by Q1 as -gm1*RL, where gm1 is the transconductance of Q1 and RL is the effective load on the collector of Q1. Since the input impedance of the "emitter follower buffer Q2" is huge, the RL in open loop is approximately 560k (RB1b), since the feedback loop is split into input and output sides via grounded C2 and RB1b appears parallel to the huge buffer impedance. Here is my simulation on the simplified circuit, showing the frequency response of Vout/Vin:



Note that the second 18k resistor affects mainly on the biasing of Q1, it does not have much significance on the gain as it is parallel to rpi2 in the small signal model. Similarly the effect of the "magical 50pF" cap is lost, because the BJT internal capacitances are most likely higher that this, resulting in a natural HF roll-off.

If someone is experiencing oscillations in this circuit, I would personally try to add a small (maybe 220 ohm) emitter resistor to Q1. This will add local negative feedback, and hopefully stabilize the high gain response of Q1.

I see the Q2 only as a buffer in this circuit. For the loop gain in closed loop state it contributes as a significant gain multiplier hfe2+1, and in the open loop it presents a high-impedance load to Q1.

[kaycee]

I've used a 100 ohm on q1 emitter to calm these down, seems to work. Just copied the idea from a Fuzz Face modification. Lower gain transistor in q1helped too, and a series resistor on the input as well. Tried a lot of things, funny beasts, some are fine right out of the blocks, others can be a pain to sort. Great fuzz though.

[PRR]

> I see the Q2 only as a buffer in this circuit.

Buffering what? Unity gain to what?

The collector impedance of Q1 is "infinite". I believe it is >10X any other impedance in the area. So the collector of Q1 is not some kind of "voltage source" that we can "unity gain" buffer.

This is however semantics. Except in that one interpretation or the other can, with reasonable simplifications, lead to incorrect answers.

I seem to have lost my sim but your gain results look similar to mine. I agree the 50pFd may be pointless, the kind of thing audio designers throw-in "just in case".

[jatalahd]

The collector impedance of Q1 is "infinite". I believe it is >10X any other impedance in the area. So the collector of Q1 is not some kind of "voltage source" that we can "unity gain" buffer.

This is correct, the collector impedance of Q1 is not only any single resistor value as I first thought it would be in the open loop state.

Buffering what? Unity gain to what?

When referring to Q2 as a buffer, I was thinking more from impedance point of view than gain. For some of us it is easier to see it that way.

[PRR]

> the collector impedance of Q1 is not only any single resistor value

A "clean process" (post-1972) BJT has collector impedance about 10,000X the emitter impedance, At 30uA, pencil near 1K at emitter and near 10Meg at collector. This is very approximate; but clearly "infinite enough" compared to all other circuit values.

[Rob Strand]

A "clean process" (post-1972) BJT has collector impedance about 10,000X the emitter impedance, At 30uA, pencil near 1K at emitter and near 10Meg at collector. This is very approximate; but clearly "infinite enough" compared to all other circuit values.

I think he is referring to the effective collector of Q1 as opposed to the collector impedance of Q1, 'ro'.
In my analysis I see that collector load directly.   In your analysis you get a different result because your BE impedance goes to the output not to ground.

Interestingly ro does have a noticeable effect.   The easiest way to see it is an AC analysis in PSPICE, plot IC and then plot gm1 * vbe1.  You will see IC is noticeably lower.  The missing current is explained by Q1's ro.

The early voltage is probably around the 70V to 100V zone,
https://en.wikipedia.org/wiki/Early_effect

In my analysis this comes about because Q1's ro is a significant fraction of the Q1's effective collector load.  In your analysis Q1's 'ro' appears across B and C of Q2.  Since your Q2 has a high gain ( ie. 80) the 'ro' gets significantly lowered due to the Miller effect  (it becomes 30k or so which is in the same order of magnitude as the Q2's BE impedances; rpi2~37k and RBE=18k).   As before these two views are identical.

It *is* noticeable effect although not dominant.

[Edit:  Occasionally you get data on 'ro'

See page 3 on,
http://www.datasheet.hk/view_online.php?id=1544966&file=0274\bc546a_421585.pdf

hoe = 1/ro  = Ic / VA

Take the "B" device

hoe = 30e-6 at 2mA  so the early voltage is VA  = 67V
]