Cute amplifier - bootstrapped

[[email protected]]

On a sunny day (Thu, 10 Jan 2013 17:31:00 -0800 (PST)) it happened
[email protected] wrote in
<[email protected]>:


mm, an ASCII magnifier?

I suppose even Google Groups has to be "good" for something.

 

[George Herold]

If I'm remembering right, I first saw that in Microwave Journal, late
'80's.  Neat stuff.  I never used it, but I found the VLF thing later
(Burhan, Radio Electronics (of all places)) and was reminded of it.

--
Cheers,
James Arthur- Hide quoted text -

- Show quoted text -

It's been with great enjoyment, that I've been reading this thread.
(Thanks all.) But I lack a complete understanding at times.
I think I've used bootstrapping only twice.
Both driving down capacitance.
In A photodiode and a driven shield in a short cable.

So my simple minded definition of bootstrapping is,
using positive feedback in just the right amount.
(Too much and you get excessive gain.)
Is that correct? Can you add more?

Thanks again, (It's great having a somewhat civil SED.)
George H.

 

[[email protected]]

It's been with great enjoyment, that I've been reading this thread.
(Thanks all.) But I lack a complete understanding at times.
I think I've used bootstrapping only twice.
Both driving down capacitance.
In A photodiode and a driven shield in a short cable.

So my simple minded definition of bootstrapping is,
using positive feedback in just the right amount.
(Too much and you get excessive gain.)
Is that correct?  Can you add more?

Bootstrapping to me means using a circuit's output to power or drive
one of its earlier stages. That is, to pull yourself up by your
bootstraps.

Here, by using the output to drive the 'high side' of R2, the voltage
seen across R2 is kept roughly the same. That, in turn, keeps i(R2)
fairly constant.

The total effect is that Q1's collector sees a current that doesn't
change much with a.c. changes in Vc(Q1). From Q1's perspective, R2 is
effectively greatly increased, which increases the stage gain
enormously.

I get g ~=23 @ 40KHz without C1, and g ~=230 with C1=22nF, a ten-fold
improvement.

Also, R2's supply can exceed Vcc, such that Q2's emitter can swing
within millivolts of the positive rail.

The real interest for me was the technique of making a resistor into a
pseudo constant-current source, simply. John did it already way back
with his ramp circuit; this was just another example.

 

[josephkk]

That's plenty cute--a three-transistor inverting amp, with feedback,
then an emitter follower, plus the bias stuff. But is there a
bootstrapped collector load in the signal chain? I didn't see it.

I didn't see any collector bootstraps either. I do see emitter bootstrap.

?-)

 

[George Herold]

Bootstrapping to me means using a circuit's output to power or drive
one of its earlier stages.  That is, to pull yourself up by your
bootstraps.

Here, by using the output to drive the 'high side' of R2, the voltage
seen across R2 is kept roughly the same.  That, in turn, keeps i(R2)
fairly constant.

The total effect is that Q1's collector sees a current that doesn't
change much with a.c. changes in Vc(Q1).  From Q1's perspective, R2 is
effectively greatly increased, which increases the stage gain
enormously.

I get g ~=23 @ 40KHz without C1, and g ~=230 with C1=22nF, a ten-fold
improvement.

Also, R2's supply can exceed Vcc, such that Q2's emitter can swing
within millivolts of the positive rail.

The real interest for me was the technique of making a resistor into a
pseudo constant-current source, simply.  John did it already way back
with his ramp circuit; this was just another example.

--
Cheers,
James Arthur- Hide quoted text -

- Show quoted text -

OK, Thanks James. I was rereading AoE on bootstrapping... similar
type 'tricks'.

George H.

 

[[email protected]]

I pondered that schematic for some time before realizing what it was.



It's some amateur's attempt at making a band-pass filter

(quasi-gyrator), but it has horrible input impedance...



Looking like 10nF in series with 16K at low frequencies,



4.4nF in series with 1.725K at 10KHz,



and -223nF at 194KHz ;-)



The shunt feedback directly to the base, driven thru the 10nF

capacitor was the clue.





...Jim Thompson

--

| James E.Thompson, CTO | mens |

| Analog Innovations, Inc. | et |

| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |

| Phoenix, Arizona 85048 Skype: Contacts Only | |

| Voice480)460-2350 Fax: Available upon request | Brass Rat |

| E-mail Icon at http://www.analog-innovations.com | 1962 |



I love to cook with wine. Sometimes I even put it in the food.

Yep- the input is garbage.

 

[[email protected]]

Thanks for not confusing us with any facts or schematics.

And thanks for not belittling us with your insightful analysis of the biasing circuit...where is that belittling insight anyway?

 

[[email protected]]

https://dl.dropbox.com/u/53724080/Circuits/AMP.JPG









--



John Larkin Highland Technology, Inc



jlarkin at highlandtechnology dot com

http://www.highlandtechnology.com



Precision electronic instrumentation

Picosecond-resolution Digital Delay and Pulse generators

Custom laser drivers and controllers

Photonics and fiberoptic TTL data links

VME thermocouple, LVDT, synchro acquisition and simulation

Okay, great, you have taken a fundamental misunderstanding of the original circuit operation to new heights

 

[Tim Williams]

Okay, great, you have taken a fundamental misunderstanding of the
original circuit operation to new heights

How so?

It's no worse than a shunt current source[1],
http://webpages.charter.net/dawill/tmoranwms/Circuits_2010/Shunt_Current_Source.png
and in fact, better than the original: no additional collector current is
required, thanks to the complement.

[1] "This revolutionary (and impressively useless) circuit is the
completion of an analogy. Consider: voltage sources are available in two
flavors, shunt (e.g., TL431) and series-pass (e.g., LM7805). But current
sources are only available in one style, series-pass. These simple
circuits complete the analogy, providing a shunt current source. In both
cases, a resistor provides a current greater than or equal to the desired
output current over the rated range; a current sense resistor, voltage
reference and voltage amplifier (VBE and a BJT in the left example; a
TL431 and differential pair in the right example) adjust a shunt current
to keep the output current constant."

Incidentally, this was drawn in 2010, so, if it were worth copyrighting,
and doesn't appear earlier elsewhere, I won.

Tim

 

[Fred Bartoli]

Tim Williams a écrit :

Okay, great, you have taken a fundamental misunderstanding of the
original circuit operation to new heights

How so?

It's no worse than a shunt current source[1],
http://webpages.charter.net/dawill/tmoranwms/Circuits_2010/Shunt_Current_Source.png
and in fact, better than the original: no additional collector current is
required, thanks to the complement.

[1] "This revolutionary (and impressively useless) circuit is the
completion of an analogy. Consider: voltage sources are available in two
flavors, shunt (e.g., TL431) and series-pass (e.g., LM7805). But current
sources are only available in one style, series-pass. These simple
circuits complete the analogy, providing a shunt current source. In both
cases, a resistor provides a current greater than or equal to the desired
output current over the rated range; a current sense resistor, voltage
reference and voltage amplifier (VBE and a BJT in the left example; a
TL431 and differential pair in the right example) adjust a shunt current
to keep the output current constant."

Incidentally, this was drawn in 2010, so, if it were worth copyrighting,
and doesn't appear earlier elsewhere, I won.

Nah, I win. I wrote about that CS back in 2005 in a thread current
source opportunely named... 'Low current source'

And it's not as useless as you might think. That one transistor CS is
very simple good trick, albeit not very accurate...

 

[[email protected]]

I wish you would try to make sense once in a while. I also wish you

would contribute something besides whining, but that's even more

unrealistic.



That last circuit is a simplification of James's bootstrap amp. It

eliminates a few parts and pushes the gain close to 60 dB.



Playing with circuits is a fun and valuable sport. Your turn.





--



John Larkin Highland Technology, Inc



jlarkin at highlandtechnology dot com

http://www.highlandtechnology.com



Precision electronic instrumentation

Picosecond-resolution Digital Delay and Pulse generators

Custom laser drivers and controllers

Photonics and fiberoptic TTL data links

VME thermocouple, LVDT, synchro acquisition and simulation

I already told you how the circuit works, and in the simplest possible terms. Your comprehension is not my problem. And you keep referring to it as James's circuit when he stated he found it on the web.

 

[josephkk]

Bootstrapping to me means using a circuit's output to power or drive
one of its earlier stages. That is, to pull yourself up by your
bootstraps.

Here, by using the output to drive the 'high side' of R2, the voltage
seen across R2 is kept roughly the same. That, in turn, keeps i(R2)
fairly constant.

The total effect is that Q1's collector sees a current that doesn't
change much with a.c. changes in Vc(Q1). From Q1's perspective, R2 is
effectively greatly increased, which increases the stage gain
enormously.

I get g ~=23 @ 40KHz without C1, and g ~=230 with C1=22nF, a ten-fold
improvement.

Also, R2's supply can exceed Vcc, such that Q2's emitter can swing
within millivolts of the positive rail.

The real interest for me was the technique of making a resistor into a
pseudo constant-current source, simply. John did it already way back
with his ramp circuit; this was just another example.

No Bout Adout it, this is an interesting circuit. Of course the much
higher voltage gain will aggravate the Miller capacitance issue hugely and
may also aggravate Early voltage issues as well.

It is an engineering tradeoff.

?-)

 

[George Herold]

[snip]

More elaborate...
http://www.analog-innovations.com/SED/MC1552LookAlike.pdf
but works nicely.  Been selling for about 50 years now >:-}  ...Jim
Thompson

On the face it may be more elaborate, but isn't rail to rail.

[snip]

It's more rail-to-rail than the OP circuit.

Like I say, anyone who thinks the OP circuit is marvy, just simulate
it, then come back and tell me about it.

Hi Jim, I'm pretty much a newb, when it comes to transistor level
design.
But the original circuit looks very similar to Figure 2.66 in AoE (2nd
ed.)
"Bootstrapping driver-stage collector load resistor in power amp."

So is it the bootstrapping you don't like or the DC biasing scheme?
(Maybe I can try to simulate it at home tonight.)
A hint might help me find the issue when simming it.

George H.
(Does simming have two m's or one? :^)

 

[Fred Bartoli]

George Herold a écrit :

On 15/01/2013 16:50, Jim Thompson wrote:
On Sun, 13 Jan 2013 13:48:15 -0700, Jim Thompson [snip]

More elaborate...
http://www.analog-innovations.com/SED/MC1552LookAlike.pdf
but works nicely. Been selling for about 50 years now >:-} ...Jim
Thompson
On the face it may be more elaborate, but isn't rail to rail.

[snip]

It's more rail-to-rail than the OP circuit.

Like I say, anyone who thinks the OP circuit is marvy, just simulate
it, then come back and tell me about it.

Hi Jim, I'm pretty much a newb, when it comes to transistor level
design.
But the original circuit looks very similar to Figure 2.66 in AoE (2nd
ed.)
"Bootstrapping driver-stage collector load resistor in power amp."

So is it the bootstrapping you don't like or the DC biasing scheme?
(Maybe I can try to simulate it at home tonight.)
A hint might help me find the issue when simming it.

George H.
(Does simming have two m's or one? :^)

It misses the 'w'

 

[[email protected]]

Man, somebody sure mangled that. Here's the original post again:

+3.3V
---
|
.-------+-------.
| |
5K R1 C1 |/ Q2
| 100n .------| 2n5089
+---||-|--. |>.
| | | |
15k R2 | '-----+------>
| | |
C2 +------' R3 820R
10n |/ Q1 |

--||-+--| 2n5089 .------+

| |>. | |
| | | R4 470R
| === | |
| | ===
'-----R5------'
220k

No, that's not the purpose. The purpose of the bootstrap is to
increase voltage gain. The input impedance doesn't matter--the input
source is a low-impedance highly capacitive transducer. This is not a
general-purpose generic IC that needs to do everything.

On the face it may be more elaborate, but isn't rail to rail.

The IC designer has luxuries the discrete designer doesn't. Jim's got
a sweet little part there, but that circuit has 13 (matched)
transistors, 3 caps and 10 resistors--not a fair comparison to a
littl' ol' two-transistor circuit.

For me, the primary purpose of bootstrapping is to ensure near rail to
rail by providing a more positive supply to the base drive of an output
device. All the other characteristics through bootstrapping, of the
amplifier circuit presented by the OP, will actually degrade performance.

Bootstrapping here increases the voltage gain and output swing--it's
cute. All two-transistor gain-of-220 amplifiers have their
limitations, naturally.

 

[[email protected]]

You sort of left that part out of your original post ;-)

True. My purpose was to explore the bootstrap, not an optimum
amplifier.

Is the "transducer" actually C2?
Yes.

Grounding the input, it is certainly
stable, with ~100° of phase margin, but barely shows 20dB of loop
gain.

LTSpice says g=220 @ 40kHz.

As a voltage amplifier it looks like a low-Q bandpass.



Awwwwh!  Shucks!  >:-}

Aww, if only discrete transistors were matched and free...

 

[[email protected]]

Try my PNP version! Less parts, more gain. It simmed at 57 dB.

I plunked a PNP (in CCS configuration) in instead of the bootstrap
cap--it almost drops right in--but I haven't matched your gain
figures.

For silliness I then a.c. bootstrapped your current source by putting
the boost cap back. Very silly!

 

[[email protected]]

The PNP follower bootstraps its own b-e resistor. That lets the NPN
run at a relatively high current (high Gm) but see a high impedance
collector load, which gets you lots of voltage gain.

Yes, I know, but the PNP's action is imperfect. So, in silly-land, I
used the cap bootstrapping trick to feed _your_ CCS from a cap-
bootstrapped CCS.

Version 4
SHEET 1 880 772
WIRE 400 -112 0 -112
WIRE 400 -80 400 -112
WIRE 0 64 0 -112
WIRE 400 64 400 0
WIRE 400 64 208 64
WIRE 208 160 208 64
WIRE 0 176 0 144
WIRE 400 176 400 64
WIRE 528 176 400 176
WIRE 608 176 528 176
WIRE 672 176 608 176
WIRE 400 240 400 176
WIRE 208 288 208 240
WIRE 336 288 208 288
WIRE 208 368 208 288
WIRE 400 400 400 336
WIRE 528 400 528 176
WIRE -224 416 -304 416
WIRE -160 416 -224 416
WIRE -16 416 -96 416
WIRE 144 416 -16 416
WIRE -304 464 -304 416
WIRE 208 512 208 464
WIRE -16 576 -16 416
WIRE 400 576 -16 576
WIRE 528 576 528 480
WIRE 528 576 400 576
WIRE -304 592 -304 544
WIRE 400 624 400 576
WIRE 400 752 400 704
FLAG 208 512 0
FLAG 400 752 0
FLAG 0 176 0
FLAG -304 592 0
FLAG 608 176 OUT
FLAG -224 416 GEN
FLAG 400 400 0
SYMBOL res 192 144 R0
WINDOW 0 -60 34 Left 2
WINDOW 3 -63 72 Left 2
SYMATTR InstName R2
SYMATTR Value 5K
SYMBOL res 512 384 R0
WINDOW 0 70 38 Left 2
WINDOW 3 60 73 Left 2
SYMATTR InstName R3
SYMATTR Value 220K
SYMBOL res 384 608 R0
WINDOW 0 -72 29 Left 2
WINDOW 3 -83 64 Left 2
SYMATTR InstName R4
SYMATTR Value 220K
SYMBOL npn 144 368 R0
WINDOW 0 98 24 Left 2
WINDOW 3 74 56 Left 2
SYMATTR InstName Q1
SYMATTR Value 2N5089
SYMBOL cap -96 400 R90
WINDOW 0 -18 31 VBottom 2
WINDOW 3 45 31 VTop 2
SYMATTR InstName C2
SYMATTR Value 10
SYMBOL voltage 0 48 R0
WINDOW 0 -107 13 Left 2
WINDOW 3 -111 51 Left 2
SYMATTR InstName V1
SYMATTR Value 3.3
SYMBOL voltage -304 448 R0
WINDOW 123 24 124 Left 2
WINDOW 39 24 152 Left 2
SYMATTR Value2 AC 0.01
SYMATTR SpiceLine Rser=1K
SYMATTR InstName V2
SYMATTR Value SINE(0 0.001 1K)
SYMBOL pnp 336 336 M180
SYMATTR InstName Q2
SYMATTR Value 2N4403
SYMBOL res 384 -96 R0
WINDOW 0 73 42 Left 2
WINDOW 3 71 76 Left 2
SYMATTR InstName R1
SYMATTR Value 5K
TEXT -272 248 Left 2 !;tran 0.01
TEXT -336 200 Left 2 !.ac oct 10 10 1meg

Cute. I was going to gripe about the output impedance, but it's
really pretty good.

Using magic SPICE transistors you can change the bias resistors from
220k to 1M, which boosts the gain even more. I wouldn't dare IRL,
natch.

 

[josephkk]

On Sun, 13 Jan 2013 13:48:15 -0700, Jim Thompson [snip]

More elaborate...

http://www.analog-innovations.com/SED/MC1552LookAlike.pdf

but works nicely. Been selling for about 50 years now >:-} ...Jim
Thompson

On the face it may be more elaborate, but isn't rail to rail.

[snip]

It's more rail-to-rail than the OP circuit.

Like I say, anyone who thinks the OP circuit is marvy, just simulate
it, then come back and tell me about it.

...Jim Thompson

Ya know what, i may simulate it to see if it is as bad as you say. It
certainly seems to have problems from what i have posted already.

?-)

 

[josephkk]

I have hundreds of DIL's with 5 matched NPN's ;-)

...Jim Thompson

Cool, what is the part number? Just the same i would like to experiment
(play) with a few if you are feeling generous this week.

?-)