Gain...

C

Cursitor Doom

Guest
Gentlemen,

When talking about common-emitter configuration BJT stages, is it
feasible to get a voltage gain of 400 in one stage or would it be better
to cascade two stages of 20? I\'d really prefer to use just one stage if
it can be done with stability. The input signal will be up to 20mV p-p
and supply voltage 12V.
cheers,

CD.
 
Cursitor Doom wrote:
Gentlemen,

When talking about common-emitter configuration BJT stages, is it
feasible to get a voltage gain of 400 in one stage or would it be better
to cascade two stages of 20? I\'d really prefer to use just one stage if
it can be done with stability. The input signal will be up to 20mV p-p
and supply voltage 12V.
cheers,

CD.
Click to expand...

That depends on the frequency of operation, the CE capacitance, the
beta, and the Early voltage. The first and last are made much easier if
you use a cascode, but that\'s two transistors already.

Plus your input swing is pretty small--that 20 mV swing will make I_C
change by a factor exp(20 mV/26 mV) = 2.2 peak-to-peak at room temperature.

With an output swing of 8 V, the quiescent bias will need to drop about
11 V from a supply of at least 15 or 16 V.

The other issue is bias stability. If you care only about the AC, you
can use nV_BE feedback to stabilize the bias. Unfortunately, that puts
an even greater premium on high beta, and multiplies the V_BE drift by
the same factor: with a voltage gain of 400, your output will drift by
400 * 2.1 mV/K = 0.84 V/K, so you\'re probably stuck with an emitter
degeneration resistor paralleled with some huge capacitor.

(If you care about the DC, you\'re pretty well screwed with either approach.)

For sufficiently slow signals, something like a 2SD2704K cascoded with a
2N3904 would probably work, if you can figure out the bias issue.

Alternatively you could try an integrated Darlington such as an MPSA14,
though that would reduce the transconductance by another factor of 2
because there are two V_BE\'s in series instead of only one.

How about a nice LM358A?

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
 
Phil Hobbs wrote:
Cursitor Doom wrote:
Gentlemen,

When talking about common-emitter configuration BJT stages, is it
feasible to get a voltage gain of 400 in one stage or would it be
better to cascade two stages of 20? I\'d really prefer to use just one
stage if it can be done with stability. The input signal will be up to
20mV p-p and supply voltage 12V.
cheers,

CD.

That depends on the frequency of operation, the
CB
capacitance, the
beta, and the Early voltage.  The first and last are made much easier if
you use a cascode, but that\'s two transistors already.

Plus your input swing is pretty small--that 20 mV swing will make I_C
change by a factor exp(20 mV/26 mV) = 2.2 peak-to-peak at room temperature.

With an output swing of 8 V, the quiescent bias will need to drop about
11 V from a supply of at least 15 or 16 V.

The other issue is bias stability.  If you care only about the AC, you
can use nV_BE feedback to stabilize the bias.  Unfortunately, that puts
an even greater premium on high beta, and multiplies the V_BE drift by
the same factor: with a voltage gain of 400, your output will drift by
400 * 2.1 mV/K = 0.84 V/K, so you\'re probably stuck with an emitter
degeneration resistor paralleled with some huge capacitor.

(If you care about the DC, you\'re pretty well screwed with either
approach.)

For sufficiently slow signals, something like a 2SD2704K cascoded with a
2N3904 would probably work, if you can figure out the bias issue.

Alternatively you could try an integrated Darlington such as an MPSA14,
though that would reduce the transconductance by another factor of 2
because there are two V_BE\'s in series instead of only one.

How about a nice LM358A?

Cheers

Phil Hobbs
Click to expand...

--
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
 
\"Phil Hobbs\" wrote in message
news:905efb96-61d6-7cdc-5d71-7639835c2390@electrooptical.net...

Cursitor Doom wrote:
Gentlemen,

When talking about common-emitter configuration BJT stages, is it
feasible to get a voltage gain of 400 in one stage or would it be better
to cascade two stages of 20? I\'d really prefer to use just one stage if
it can be done with stability. The input signal will be up to 20mV p-p
and supply voltage 12V.
Click to expand...

Realistically, one really needs two stages with this sort of spec.

cheers,

CD.

That depends on the frequency of operation, the CE capacitance, the beta,
and the Early voltage. The first and last are made much easier if you use
a cascode, but that\'s two transistors already.
Click to expand...

Well.....

beta really has only a minor effect on the actual flatband gain, it\'s a
second order effect.

The Early voltage will only matter if the biasing load is a current source.
Its usually swamped by the collector load resistor.

The inherent basic gain, with a collector resistor, is given by

Av = 40 x Vdcr

Where Vdcr is the voltage across the collector load resistor

This is explained here:

https://www.kevinaylward.co.uk/ee/bipolardesign1/bipolardesign1.xht

If there is an ideal current source, the limit of gain is

Av = Ve/25mv = 40 x Ve

Plus your input swing is pretty small--that 20 mV swing will make I_C
change by a factor exp(20 mV/26 mV) = 2.2 peak-to-peak at room temperature.
Click to expand...

This is a large input signal if not degenerated by an emitter resistor,
which will lower the gain.

The notionally distortion of a basic stage is:

%nd distortion = Vi in mv

so, a 20 mv signal is heading to around 100% 2nd harmonic

This is explained here:

https://www.kevinaylward.co.uk/ee/bipolardesign2/bipolardesign2.xht

One only wants to drive a raw transistor stage with < 5mV

For a diff pair, the even distortion cancels, thus one might get around 50
mV signal handling

With an output swing of 8 V, the quiescent bias will need to drop about 11
V from a supply of at least 15 or 16 V.

The other issue is bias stability. If you care only about the AC, you can
use nV_BE feedback to stabilize the bias. Unfortunately, that puts an even
greater premium on high beta, and multiplies the V_BE drift by the same
factor: with a voltage gain of 400, your output will drift by 400 * 2.1
mV/K = 0.84 V/K, so you\'re probably stuck with an emitter degeneration
resistor paralleled with some huge capacitor.
Click to expand...

Yep. beta is a tad of a problem in setting up a bias.

There are ways around this, but it all adds up to more transistors.

https://www.kevinaylward.co.uk/ee/bipolardesign3/bipolardesign3.xht

(If you care about the DC, you\'re pretty well screwed with either
approach.)

For sufficiently slow signals, something like a 2SD2704K cascoded with a
2N3904 would probably work, if you can figure out the bias issue.

Alternatively you could try an integrated Darlington such as an MPSA14,
though that would reduce the transconductance by another factor of 2
because there are two V_BE\'s in series instead of only one.

How about a nice LM358A?
Click to expand...

Yep......

Why anyone would use a discrete transistor stage today is indeed a
mystery...


-- Kevin Aylward

http://www.anasoft.co.uk/ SuperSpice
http://www.kevinaylward.co.uk/ee/index.html
 
Kevin Aylward wrote:
\"Phil Hobbs\"  wrote in message
news:905efb96-61d6-7cdc-5d71-7639835c2390@electrooptical.net...

How about a nice LM358A?

Yep......

Why anyone would use a discrete transistor stage today is indeed a
mystery...
Click to expand...

You need to get out more. ;)

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 Thu, 30 Dec 2021 15:47:19 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

Cursitor Doom wrote:
Gentlemen,

When talking about common-emitter configuration BJT stages, is it
feasible to get a voltage gain of 400 in one stage or would it be better
to cascade two stages of 20? I\'d really prefer to use just one stage if
it can be done with stability. The input signal will be up to 20mV p-p
and supply voltage 12V.
cheers,

CD.

That depends on the frequency of operation, the CE capacitance, the
beta, and the Early voltage. The first and last are made much easier if
you use a cascode, but that\'s two transistors already.

Plus your input swing is pretty small--that 20 mV swing will make I_C
change by a factor exp(20 mV/26 mV) = 2.2 peak-to-peak at room temperature.

With an output swing of 8 V, the quiescent bias will need to drop about
11 V from a supply of at least 15 or 16 V.

The other issue is bias stability. If you care only about the AC, you
can use nV_BE feedback to stabilize the bias. Unfortunately, that puts
an even greater premium on high beta, and multiplies the V_BE drift by
the same factor: with a voltage gain of 400, your output will drift by
400 * 2.1 mV/K = 0.84 V/K, so you\'re probably stuck with an emitter
degeneration resistor paralleled with some huge capacitor.

(If you care about the DC, you\'re pretty well screwed with either approach.)

For sufficiently slow signals, something like a 2SD2704K cascoded with a
2N3904 would probably work, if you can figure out the bias issue.

Alternatively you could try an integrated Darlington such as an MPSA14,
though that would reduce the transconductance by another factor of 2
because there are two V_BE\'s in series instead of only one.

How about a nice LM358A?
Click to expand...

HaHa! That would be too easy, Phil. :)
I omitted some key paramters in the original posting as I wanted to
just keep it general. The input signal will be 50Hz to 10Khz. I don\'t
know exactly the source impedance but it will be around 600-800 ohms.
Distortion is (unusally) not a major consideration here.
The first attempt I made from hand calcs was very current frugal
(battery power in practice so need to keep it down) and when finished
only measured 0.01% THD. But it produced an output of ~1V p-p and
wasn\'t readily adjustable to a more useful level up or down without
screwing up the quiescent current draw. Nothing\'s ever as simple as it
seems at first, clearly. :-/
 
Cursitor Doom wrote:
On Thu, 30 Dec 2021 15:47:19 -0500, Phil Hobbs
snip
How about a nice LM358A?

HaHa! That would be too easy, Phil. :)
I omitted some key parameters in the original posting as I wanted to
just keep it general. The input signal will be 50Hz to 10Khz. I don\'t
know exactly the source impedance but it will be around 600-800 ohms.
Distortion is (unusually) not a major consideration here.
The first attempt I made from hand calcs was very current frugal
(battery power in practice so need to keep it down) and when finished
only measured 0.01% THD. But it produced an output of ~1V p-p and
wasn\'t readily adjustable to a more useful level up or down without
screwing up the quiescent current draw. Nothing\'s ever as simple as it
seems at first, clearly. :-/
Click to expand...

To get an 8 V output swing at a gain of 400, your poor transistor\'s
Early voltage would have to be, like, 3200 V, not counting the actual
collector resistor.

A cascode might work, apart from the Class-A current drain problem
mentioned upthread.

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 Thu, 30 Dec 2021 21:34:27 -0000, \"Kevin Aylward\"
<kevinRemoveandReplaceATkevinaylward.co.uk> wrote:
Why anyone would use a discrete transistor stage today is indeed a
mystery...


-- Kevin Aylward

http://www.anasoft.co.uk/ SuperSpice
http://www.kevinaylward.co.uk/ee/index.html
Click to expand...

Some of the RF people are ashamed to admit they are selling
transistors. They label the pins RF IN and RF OUT and GROUND.

Sometimes a transistor is just what you need.

--

If a man will begin with certainties, he shall end with doubts,
but if he will be content to begin with doubts he shall end in certainties.
Francis Bacon
 
On Thu, 30 Dec 2021 16:03:48 -0800, John Larkin
<jlarkin@highland_atwork_technology.com> wrote:

On Thu, 30 Dec 2021 21:34:27 -0000, \"Kevin Aylward\"
kevinRemoveandReplaceATkevinaylward.co.uk> wrote:

Why anyone would use a discrete transistor stage today is indeed a
mystery...


-- Kevin Aylward

http://www.anasoft.co.uk/ SuperSpice
http://www.kevinaylward.co.uk/ee/index.html

Some of the RF people are ashamed to admit they are selling
transistors. They label the pins RF IN and RF OUT and GROUND.
Click to expand...

Seriously??
<boggle>

Sometimes a transistor is just what you need.
Click to expand...
 
John Larkin wrote:
On Thu, 30 Dec 2021 21:34:27 -0000, \"Kevin Aylward\"
kevinRemoveandReplaceATkevinaylward.co.uk> wrote:

Why anyone would use a discrete transistor stage today is indeed a
mystery...


-- Kevin Aylward

http://www.anasoft.co.uk/ SuperSpice
http://www.kevinaylward.co.uk/ee/index.html

Some of the RF people are ashamed to admit they are selling
transistors. They label the pins RF IN and RF OUT and GROUND.

Sometimes a transistor is just what you need.

Yup, particularly in front ends.
Click to expand...

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
 
To get an 8 V output swing at a gain of 400, your poor transistor\'s
Early voltage would have to be, like, 3200 V, not counting the actual
collector resistor.
Click to expand...

That is possible to find.
ZXTN2018F Va = 4600
tho\' beta is low, 100@10mA
Ref: AoE3, pg. 501
 
On Thursday, 30 December 2021 at 20:17:58 UTC, Cursitor Doom wrote:
Gentlemen,

When talking about common-emitter configuration BJT stages, is it
feasible to get a voltage gain of 400 in one stage or would it be better
to cascade two stages of 20? I\'d really prefer to use just one stage if
it can be done with stability. The input signal will be up to 20mV p-p
and supply voltage 12V.
cheers,

CD.
Click to expand...

400 is possible but difficult. You can get far more if determined. How about a millioon?
https://groups.google.com/g/sci.electronics.design/c/RR4WL714yLU
 
Rich S wrote:
To get an 8 V output swing at a gain of 400, your poor transistor\'s
Early voltage would have to be, like, 3200 V, not counting the actual
collector resistor.

That is possible to find.
ZXTN2018F Va = 4600
tho\' beta is low, 100@10mA
Ref: AoE3, pg. 501
Click to expand...

But to get a high enough gain, even with typical values, the collector
load would have to be very large--about triple.

1/(1/3200 -1/4600) =~ 10500.

A cascode fixes this very handily.

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 Thursday, December 30, 2021 at 8:17:58 PM UTC, Cursitor Doom wrote:
Gentlemen,

When talking about common-emitter configuration BJT stages, is it
feasible to get a voltage gain of 400 in one stage or would it be better
to cascade two stages of 20? I\'d really prefer to use just one stage if
it can be done with stability. The input signal will be up to 20mV p-p
and supply voltage 12V.
cheers,

CD.
Click to expand...

Hi CD,
If it were me, I\'d grab a National Semiconducor
Linear Applications handbook, look at AN-222.
While its main topic is using the LM394, the
FIGURE 4 is fairly relevant to your project.
For the low-noise NPN, the AoE has table
with many to choose from.
cheers, RS
 
Rich Schmuck wrote:
================
CD.

Hi CD, > If it were me, I\'d grab a National Semiconducor
Linear Applications handbook, look at AN-222.
While its main topic is using the LM394, the
FIGURE 4 is fairly relevant to your project.
Click to expand...

** The LM394 is long obsolete.

NOS examples go for $40 on Ebay.

CD is a massive troll and a moron - so are you.



..... Phil
 
On Thu, 30 Dec 2021 18:19:27 -0800 (PST), Rich S
<richsulinengineer@gmail.com> wrote:

On Thursday, December 30, 2021 at 8:17:58 PM UTC, Cursitor Doom wrote:
Gentlemen,

When talking about common-emitter configuration BJT stages, is it
feasible to get a voltage gain of 400 in one stage or would it be better
to cascade two stages of 20? I\'d really prefer to use just one stage if
it can be done with stability. The input signal will be up to 20mV p-p
and supply voltage 12V.
cheers,

CD.

Hi CD,
If it were me, I\'d grab a National Semiconducor
Linear Applications handbook, look at AN-222.
While its main topic is using the LM394, the
FIGURE 4 is fairly relevant to your project.
For the low-noise NPN, the AoE has table
with many to choose from.
cheers, RS
Click to expand...

I will; many thanks.
 
On 12/30/21 10:12 PM, Phil Allison wrote:
Rich Schmuck wrote:
================
CD.

Hi CD, > If it were me, I\'d grab a National Semiconducor
Linear Applications handbook, look at AN-222.
While its main topic is using the LM394, the
FIGURE 4 is fairly relevant to your project.

** The LM394 is long obsolete.

NOS examples go for $40 on Ebay.

CD is a massive troll and a moron - so are you.



.... Phil
Click to expand...

Would you be interested in a Latvian equivalent:

<https://www.ericasynths.lv/shop/ics/as394-matched-transistors/>

Looks like \"ALFA RPAR\" specializes in repros of some ICs that were in
vintage synthesizers and effects
 
On Friday, December 31, 2021 at 5:54:11 PM UTC, bitrex wrote:
On 12/30/21 10:12 PM, Phil Allison wrote:
Rich Schmuck wrote:
================
CD.

Hi CD, > If it were me, I\'d grab a National Semiconducor
Linear Applications handbook, look at AN-222.
While its main topic is using the LM394, the
FIGURE 4 is fairly relevant to your project.

** The LM394 is long obsolete.

NOS examples go for $40 on Ebay.

CD is a massive troll and a moron - so are you.



.... Phil

Would you be interested in a Latvian equivalent:

https://www.ericasynths.lv/shop/ics/as394-matched-transistors/

Looks like \"ALFA RPAR\" specializes in repros of some ICs that were in
vintage synthesizers and effects
Click to expand...

Cool! So Latvia rocks.
Only € 2.70 - not bad at all.


Aside from LM394..
Other new-stock matched BJT duals or arrays live on...

http://thatcorp.com/300-series_Matched_Transistor_Array_ICs.shtml
https://www.analog.com/en/parametricsearch/10988#/
https://www.onsemi.com/products/discrete-power-modules/general-purpose-and-low-vcesat-transistors/nst45011mw6t1g
 
On Thu, 30 Dec 2021 19:11:41 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

John Larkin wrote:
On Thu, 30 Dec 2021 21:34:27 -0000, \"Kevin Aylward\"
kevinRemoveandReplaceATkevinaylward.co.uk> wrote:

Why anyone would use a discrete transistor stage today is indeed a
mystery...


-- Kevin Aylward

http://www.anasoft.co.uk/ SuperSpice
http://www.kevinaylward.co.uk/ee/index.html

Some of the RF people are ashamed to admit they are selling
transistors. They label the pins RF IN and RF OUT and GROUND.

Sometimes a transistor is just what you need.

Yup, particularly in front ends.
Click to expand...

FETs are better in that role IMHO.
 
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