( Repost, apparently the first one didn't make it!? )
Right, it's a slick single-stage amplifier,
compared to Phil's two-stage amplifier.
I was trying to avoid even looking at those
silly resistors in series with the opamp inputs.
But lets drop them in. R4a and R4b below.
-------+-------------+---Vs
| |
\ \ R3b
/R2a R2b/ +-/\/\---+-->Vout
\ \ | |
| | R4b | _ |
| +--/\/\---+--|- \ |
| | Vx | >-+
+-------------|--/\/\---+--|+_/
| | R4a |
\|/I1 I2\|/ \
| | /R3a
|/ \| \
Vin ---|npn npn|----. |
|\e e/| | |
0v--- | R1 | ---+--+---0v
+----/\/\-----+
| |
\|/ I I \|/
Constant current
Doing similar sums as before, I get...
Vout/Vin = -2(R3/R1)*( R2/(R2+R4) ).
Now put some spin on it..... Call R4 = K*R2.
Vout/Vin = -2(R3/R1)*( 1/(1+K) ).
Tony, thanks loads, for taking the time to put up
an ASCII drawing, where everyone can be on the same
page in the discussion. This business of posting
web page links, or placing pdfs on a.b.s.e., etc.,
simply does not lend itself well to a conversation.
I imagine one of Jim's criticisms of the circuit,
I'm guessing since he refused to spell it out, is
that the R4 resistors take away from the potential
gain of the overall amplifier, or, for a set gain,
take away loop gain that could be used to reduce
distortion, etc. Your first (top) drawing without
the R4 resistors is better, but it still suffers
from the fact that the R2 resistors will have to
be smaller than the R3 resistors, shunting current.
Even though R2 doesn't appear in the gain equation,
they are degrading the performance. One solution
is to replace R2 with current sources, Ix. These
would have to be servo'd to match I1, with another
opamp looking at the average value of Vx compared
to some bias voltage, etc.
servo'd constant current
| | R3b
\|/ Ix \|/ ,-/\/\---+-->Vout
| | | _ |
| +---------+--|- \ |
| | | >-'
Vx---> +-------------|---------+--|+_/
| | |
\|/I1 I2\|/ \
| | /R3a
|/ \| \
Vin ---| npn npn |----. |
|\e e/| | |
0v--- | R1 | ---+--+---0v
+----/\/\-----+
| |
\|/ I I \|/
Constant current
This would be a bit of a mess, for an arguable
improvement, but possibly not something an IC
designer, who generally has lots of silicon
available, would shy away from.
Now, to change the subject to an item I'm not at
all happy with, the huge ugly electrolytic in the
input-pair emitter path, see C1 below.
-------+-------------+---Vs
| | G = 2 R3 / R1
\ \
/R2a R2b/ R3b
\ \ ,-/\/\---+-->Vout
| | | _ |
| +---------+--|- \ |
| | | >-'
Vx---> +-------------|---------+--|+_/
| | |
| | \
| Q1 Q2 | /R3a
|/ \| \
--+ --| npn npn |--. |
Vin | |\e e/| | |
--+-|-- | R1 C1 | --' -+---0v
| | +--/\/\--||---+
\ \ | | Q1 Q2 composite
/ / \|/ I I \|/ Sziklai pairs
| | Constant currents
--+-+-0V
C1 is deemed necessary to prevent the offset voltage
of the input NPN transistors, which could be 25mV or
even 50mV, from saturating the output at G = 1000.
R1 is only 22 ohms at the highest gain, so C1 has to
be large, 1000uF for a 7Hz rolloff, in Phil's design,
http://sound.westhost.com/project66.htm
But C1 is trouble for several reasons. One problem
is the distortion created from the high dielectric
absorption for electrolytics. I've read that this
is an issue for frequencies up to 10x away from the
-3dB frequency, which implies degradation up to 70Hz
at the highest gain. I'd like to eliminate C1, and
rely on the set of C2 capacitors at the opamp input
to deal with the offset voltage. Phil used 47uF
electrolytics, which gives a 1.5Hz rolloff compared
to his R2 = 2.2k resistors, which nicely moves the
10x distortion region down to 15Hz.
And as a bonus, the 100ms time constant is fixed,
independent of gain. This compares to C1 R1, which
varied from 22ms to 10 seconds over the gain range.
-------+-------------+---Vs
| | G = 2 R3 / R1
\ \
/R2a R2b/ R3b
\ \ ,-/\/\---+-->Vout
| | C2b | _ |
| Vy--> +----||---+--|- \ |
| | C2a | >-'
Vx---> +-------------|----||---+--|+_/
| | + - |
| | \
| Q1 Q2 | /R3a
|/ \| \
--+ --| npn npn |--. |
Vin | |\e e/| | |
--+-|-- | R1 | --' -+---0v
| | +----/\/\-----+
\ \ | | Q1 Q2 composite
/ / \|/ I I \|/ Sziklai pairs,
| | Constant currents matched offsets
--+-+-0V
I'd solve the dc-offset problem simply by selecting
Q1 and Q2 for a maximum offset voltage of say 5mV.
The input transistors are cheap, easily allowing
for a small pile to choose from, and they should be
checked for noise anyway, before use.* If we assume
a 10mV worst-case input offset, this results in a
modest +/- 1-volt deviation from nominal at points
Vx and Vy. With C2 in place, that's acceptable.
What's more, thanks to the use of Sziklai pairs,
the operating current of each input transistor is
unaffected, because the current change is taken up
by the second transistor in each Sziklai pair.
So that's three parts taken from Phil's microphone
amplifier, arguably for an improved performance.
* Reading in the forum suggested by Martin Griffith
http://www.prodigy-pro.com/forum/viewtopic.php?t=20038
some in the studio pro-audio crowd like the 2n4403
pnp transistor for low-noise input stages (most of
us instrumentation engineers prefer other parts).
Apparently Motorola made very quiet ones in the good
old days, and Fairchild's parts are pretty good now.
But the 2n4403 isn't specified or suggested for low
noise, and surely each one should be individually
vetted for such use.
