Cute amplifier - bootstrapped

[[email protected]]

Adapted from something I saw on the web...

I had to scratch my head until I understood C1: it bootstraps Q1's
collector load (R2) impedance, turning it effectively into a current
source. Gain is accordingly greatly increased. G = ~220 @ 40KHz.

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

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

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

R3-R4 set the d.c. output level.

Gain is component-sensitive, but the bootstrap is a cute technique.
That with feedback could stabilize the gain.

 

[[email protected]]

Commonly done in class-B output stages to get closer to rail-to-rail
behavior.

That's another advantage.

 

[[email protected]]

Adapted from something I saw on the web...



I had to scratch my head until I understood C1: it bootstraps Q1's

collector load (R2) impedance, turning it effectively into a current

source. Gain is accordingly greatly increased. G = ~220 @ 40KHz.



+3.3V

---

|

.-------+-------.

| |

5K R1 C1 |/ Q2

| 100n .------| 2n5089

+---||-|--. |>.

| | | |

15k R2 | '-----+------>

| | |

C2 +------' R3 820R

10n |/ Q1 |


| |>. | |

| | | R4 470R

| === | |

| | ===

'-----R5------'

220k



R3-R4 set the d.c. output level.



Gain is component-sensitive, but the bootstrap is a cute technique.

That with feedback could stabilize the gain.



--

Cheers,

James Arthur

You need to turn that around, it is the emitter follower that is being bootstrapped and not the CE. So the CE + CC composite looks like the classic very high gain single pole amplifier inside the feedback loop formed by the resistor shunt feedback divider, everything is simplified. See the textbook write-up here, section 1.17.2:
http://books.google.com/books?id=1-...epage&q=bootstrapped emitter follower&f=false

 

[[email protected]]

https://dl.dropbox.com/u/53724080/Circuits/Current_Sources/Isrc_5.JPG

How did that eventually work out?

 

[[email protected]]

The output impedance of the follower is probably a few tens of ohms.

For small signals it doesn't even notice the extra loading on its emitter..



You can bootstrap a follower, in fact you have to if you want the

bootstrap itself to work properly, but that involves doing something

with its collector.

Huh? You bootstrap the follower to increase its input impedance, not lower its output impedance.

 

[[email protected]]

Bootstrapping rocks. This thing improves the constant-current-ness of
a linear ramp generator at higher speeds....

https://dl.dropbox.com/u/53724080/Circuits/Current_Sources/Isrc_5.JPG

I remember that trick of yours. Fun.

Phil does some uber-bootstrapped stuff with jfet front ends. He sort
of bootstraps the entire planet, or the entire universe, all around a
BF862.

One of the neatest I've seen was a VLF JFET amplifier. It used
transformers to bootstrap its input capacitance to nada.

James

 

[[email protected]]

Except for the Early effect in Q1.

I made a cascode version, but that was getting silly. A dual op-amp
makes more sense at that point with fewer parts.

Talking about silly, a little positive feedback really makes it
sing...

+3.3V
---
|
.-------+-------.
| |
5K R1 C1 |/ Q2
| 22n .------| 2n5089
+---||-|--. |>.
| | | |
15k R2 | '-----+------>
| | |
C2 +------' R3 820R
10n |/ Q1 |

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

| |>. | |
| | | R4 470R
| | | |
| +---------------'
| | |
| R6 3r3 |
| | |
| === |
| |
'-----R5------'
220k

G= 1050 @ 40KHz.

There actually is some negative feedback via R5--that stabilizes the
gain a little bit.

 

[[email protected]]

Except for the shunt-feedback >:-}

The whole thing is a bit crappy.

Here's how to do it...

     http://www.analog-innovations.com/SED/MC1552-DataSheet.pdf

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.

 

[[email protected]]

<weird double spaced stuff snipped>



The follower's input impedance is limited by its beta and Early voltage

at low frequency, and its C_cb at high frequency. Beta you can't do too

much about, but Early and C_cb you can fix by bootstrapping its

collector. You can't improve the SNR, but you can make the frequency

response and Zin much prettier.

Okay, those are fine points, but in this case the designer is trying to eliminate the shunt effect of the emitter follower input bias resistor network, and the bootstrapping succeeds well there.

 

[[email protected]]

That's sort of a strange way to look at it. The common-emitter stage

wouldn't do much without those resistors, so I'd identify them as the

collector load of the first stage rather than an input network for the

second stage.

I believe that is how that circuit is put together. Whatever the application was, they knew they needed controlled gain with buffering for the loading.. So someone figured out they can use the super high midband impedance of the bootstrapped CC as a super high gain setting loading for the CE, all of which makes for an easily compensated relatively accurate feedback amplifier.

 

[[email protected]]

You need to turn that around, it is the emitter follower that is being bootstrapped and not the CE. So the CE + CC composite looks like the classic very high gain single pole amplifier inside the feedback loop formed by theresistor shunt feedback divider, everything is simplified. See the textbook write-up here, section 1.17.2:http://books.google.com/books?id=1-jiL0s8y7EC&pg=PA78&lpg=PA78&dq=boo...

Yep, that's a classic way to raise a.c. input impedance.

ISTM there's really no distinction compared to describe it as
bootstrapping the collector load. The collector load R2, being in
parallel with the Q2 input impedance, bootstrapping R2 raises QA2's
input impedance, that's true. But, the key effect in this circuit is
raising the first stage gain, so it seems most appropriate to look at
it from that perspective.

Either / or.

 

[[email protected]]

Yep, that's a classic way to raise a.c. input impedance.



ISTM there's really no distinction compared to describe it as

bootstrapping the collector load. The collector load R2, being in

parallel with the Q2 input impedance, bootstrapping R2 raises QA2's

input impedance, that's true. But, the key effect in this circuit is

raising the first stage gain, so it seems most appropriate to look at

it from that perspective.



Either / or.



--

Cheers,

James Arthur

Okay, look at it under a microscope if you prefer it that way.

 

[[email protected]]

In the tube and early-transistor days, gain was expensive, so positive-feedback
tricks, and things like transformer coupling, were more common.

Yep. Jim William's description of the Philbrick varactor amp inspired
R6 above. IRL Jim's throwing a handful of transistors at it makes
more sense for an IC designer--less component-sensitive--but it's
always fun to play.

As mentioned before, I'll likely use a dual op-amp instead, cascaded
inverting stages, a.c.-coupled, d.c. offset. That's one op-amp, two
caps, six resistors--boring, reliable, and repeatable.

 

[[email protected]]

The output impedance of the follower is probably a few tens of ohms.
For small signals it doesn't even notice the extra loading on its emitter..

Q2's input resistance is gain(Q2) x (R3+R4) anyhow, which is on the
order of half a megohm.

So, Q2's un-bootstrapped input impedance doesn't limit Q1's gain, R2
does. Boosting Q2's input impedance per se wouldn't help.

 

[[email protected]]

Not to thread-drift or anything (who, me?) but similar toplogies are, if
possible, even more ancient than JT. This is the once-famous "GE circuit",
widely used for phono and tape preamps.

https://dl.dropbox.com/u/53724080/Circuits/GEcircuit.jpg

I wouldn't be surprised if there were a tube ancestor.

I've seen that ages ago, a faded memory, I think likely from that same
G.E. databook. Yours is much linear-er than the circuit I posted.

Hmm. Could even bootstrap the first collector load. Only drawback is
possibly a limited output swing compared to the emitter-follower Q2
version...

I might goof around thinking about that.


James

 

[[email protected]]

Not to thread-drift or anything (who, me?) but similar toplogies are, if

possible, even more ancient than JT. This is the once-famous "GE circuit",

widely used for phono and tape preamps.



https://dl.dropbox.com/u/53724080/Circuits/GEcircuit.jpg

That's not even remotely close. That is the two transistor job with superstablized Q-point self-bias and series feedback.

 

[[email protected]]

Gosh, really?

Yep- really- and that circuit has come up as a topic before. You couldn't get to square one figuring out the DC bias then IIRC.

 

[[email protected]]

Thanks for not confusing us with any facts or schematics.

You're very welcome.

 

[[email protected]]

Thanks for not confusing us with any facts or schematics.

Fred's talking about "another bizarre audio circuit", 3/2/2011

https://groups.google.com/group/sci...read/973bb460733ddab7/67d85dc141f9c4ef?hl=en&



James

 

[[email protected]]

You can combine that with noise resistance matching too, which is a huge
win.

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.