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Stable simple dc amplifiers

I need a simple(=3D cheap) relatively stable(temp and noise) dc amplifier t= hat can take a voltage from 0 to 5V and output from around 0V(not critical)= to near Vcc(but arbitrary) somewhat linearly.

I'm thinking a simple bjt amplifier with temp compensation will work decent= ly but the issue is linearity and range.

e.g., the output voltage of an ideal CE amplifier is Vout =3D Vcc - Rc/Re*V= in

Of course when Vin =3D 0 volts, Vout =3D Vcc. When Vin is 5V we get Vout = =3D Vcc - 5Rc/Re but we would like 0V or some low fixed voltage instead.=20

Adding temperature compensation makes things worse since it effects our upp= er range(which is more crucial than the lower range) since Vin cannot swing= down to 0V(I'm assuming the simple diode compensation scheme generally use= d).

In any case the requirements are

  1. Relatively cheap and easy to built(e.g., a few discrete components)/
  2. Amplifies a voltage from [0, Vin_max] to approximately [0, Vcc] with the= upper range being more important. Vcc is somewhat arbitrary =3D=3D> May ch= ange after the design of the circuit =3D=3D> no component values can depend= on Vcc to achieve specs. (Obviously one can assume that Vcc is within all = the maximum voltage ratings of the components)
  3. Temperature stable/compensated. The temperature range will vary only aro= und 10-20C.
  4. Relatively low noise(not that big of an issue since caps can take care o= f the big problems).

Vin ranges from 0 to 5V and Vcc ranges from about 50V to 500V.

Reply to
Archival
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can take a voltage from 0 to 5V and output from around 0V(not critical) to near Vcc(but arbitrary) somewhat linearly.

but the issue is linearity and range.

5Rc/Re but we would like 0V or some low fixed voltage instead.

range(which is more crucial than the lower range) since Vin cannot swing down to

0V(I'm assuming the simple diode compensation scheme generally used).

upper range being more important. Vcc is somewhat arbitrary ==> May change after the design of the circuit ==> no component values can depend on Vcc to achieve specs. (Obviously one can assume that Vcc is within all the maximum voltage ratings of the components)

10-20C.

the big problems).

Why not an opamp?

--

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www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
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Reply to
John Larkin

can take a voltage from 0 to 5V and output from around 0V(not critical) to near Vcc(but arbitrary) somewhat linearly.

but the issue is linearity and range.

5Rc/Re but we would like 0V or some low fixed voltage instead.

range(which is more crucial than the lower range) since Vin cannot swing down to

0V(I'm assuming the simple diode compensation scheme generally used).

upper range being more important. Vcc is somewhat arbitrary ==> May change after the design of the circuit ==> no component values can depend on Vcc to achieve specs. (Obviously one can assume that Vcc is within all the maximum voltage ratings of the components)

10-20C.

the big problems).

There are a lot of opamps that can fill the bill..put a FET at the output and use some feedback; gain of 100 good for 0-5V in, 0-500V out.

Reply to
Robert Baer

can take a voltage from 0 to 5V and output from around 0V(not critical) to near Vcc(but arbitrary) somewhat linearly.

but the issue is linearity and range.

5Rc/Re but we would like 0V or some low fixed voltage instead.

range(which is more crucial than the lower range) since Vin cannot swing down to

0V(I'm assuming the simple diode compensation scheme generally used).

upper range being more important. Vcc is somewhat arbitrary ==> May change after the design of the circuit ==> no component values can depend on Vcc to achieve specs. (Obviously one can assume that Vcc is within all the maximum voltage ratings of the components)

10-20C.

the big problems).

I assume you want discrete because of the high Vcc?

I think you will need an opamp as a "front-end" since it will be hard to get good accuracy from discretes. Transistors on opamps are all matched.

So an opamp followed by a discrete booster stage with overall DC feedback to keep it accurate.

--

John Devereux
Reply to
John Devereux

that can take a voltage from 0 to 5V and output from around 0V(not critica= l) to near Vcc(but arbitrary) somewhat linearly.

ntly but the issue is linearity and range.

*Vin

=3D Vcc - 5Rc/Re but we would like 0V or some low fixed voltage instead.=20

pper range(which is more crucial than the lower range) since Vin cannot swi= ng down to 0V(I'm assuming the simple diode compensation scheme generally u= sed).

he upper range being more important. Vcc is somewhat arbitrary =3D=3D> May = change after the design of the circuit =3D=3D> no component values can depe= nd on Vcc to achieve specs. (Obviously one can assume that Vcc is within al= l the maximum voltage ratings of the components)

round 10-20C.

of the big problems).

Your requirements make no sense since Vcc varies over 10:1 range how could = the minimum Vcc-5Rc/Re not vary as well. If you get Vout,min=3D0V with Vcc= =3D50V then Vout,min will be 450V for Vcc=3D50V.

Reply to
bloggs.fredbloggs.fred

around 10-20C.

it so.

The required gain is a function of Vcc. That's where all the ugly is.

Reply to
Ralph Barone

amplifier that can take a voltage from 0 to 5V and output from around =

0V(not critical) to near Vcc(but arbitrary) somewhat linearly.

decently but the issue is linearity and range.

Rc/Re*Vin

Vout =3D Vcc - 5Rc/Re but we would like 0V or some low fixed voltage = instead.=20

our upper range(which is more crucial than the lower range) since Vin = cannot swing down to 0V(I'm assuming the simple diode compensation scheme= generally used).

components)/

with the upper range being more important. Vcc is somewhat arbitrary = =3D=3D> May change after the design of the circuit =3D=3D> no component = values can depend on Vcc to achieve specs. (Obviously one can assume that= Vcc is within all the maximum voltage ratings of the components)

only around 10-20C.

care of the big problems).

to

matched.

amplifier that can take a voltage from 0 to 5V and output from around =

0V(not critical) to near Vcc(but arbitrary) somewhat linearly.

decently but the issue is linearity and range.

Rc/Re*Vin

Vout =3D Vcc - 5Rc/Re but we would like 0V or some low fixed voltage = instead.=20

our upper range(which is more crucial than the lower range) since Vin = cannot swing down to 0V(I'm assuming the simple diode compensation scheme= generally used).

components)/

with the upper range being more important. Vcc is somewhat arbitrary = =3D=3D> May change after the design of the circuit =3D=3D> no component = values can depend on Vcc to achieve specs. (Obviously one can assume that= Vcc is within all the maximum voltage ratings of the components)

only around 10-20C.

care of the big problems).

to

matched.

hoping someone would have a simple way to do this. Accuracy is not the = main issue here.

or is too noisy then it shouldn't be a problem. As long as the problems = can be reduced through capacitance I don't see any issue.

like any others. Using a CE driving a CC does the job theoretically = excluding the range mapping issues.

Seems like it may not be the safest thing to do and may add extra cost = and complexity when a simple BJT might work. I've seen a lot of tricks = with BJT amplifiers and I'm hoping there maybe one for my case.

Oh hell, it can be done easily enough and safely enough. It is just that partial opamp solutions are usually easier, cheaper and more effective done correctly.

?-)

Reply to
josephkk

amplifier that can take a voltage from 0 to 5V and output from around =

0V(not critical) to near Vcc(but arbitrary) somewhat linearly.

work decently but the issue is linearity and range.

- Rc/Re*Vin

get Vout =3D Vcc - 5Rc/Re but we would like 0V or some low fixed voltage = instead.=20

effects our upper range(which is more crucial than the lower range) since= Vin cannot swing down to 0V(I'm assuming the simple diode compensation = scheme generally used).

components)/

with the upper range being more important. Vcc is somewhat arbitrary = =3D=3D> May change after the design of the circuit =3D=3D> no component = values can depend on Vcc to achieve specs. (Obviously one can assume that= Vcc is within all the maximum voltage ratings of the components)

only around 10-20C.

take care of the big problems).

could the minimum Vcc-5Rc/Re not vary as well. If you get Vout,min=3D0V = with Vcc=3D50V then Vout,min will be 450V for Vcc=3D50V.

voltage and has nothing to do with the output voltage.

operation and this is not the case.

specifically specified. In the real circuit it might be 234.32V but will = not change in that circuit. What I do know is that it will be between 50V= and 450V. If I can design a circuit that works over the whole range then= I won't have to worry about changing resistors.

change it to be between 50V and 450V. I would like to design the circuit = so it behaves the same regardless without having to change resistors to = make it so.

Bloody hell. What is the output current and is there any accuracy specification? How do expect to design, let alone build, anything so piss poorly specified

/:-((

Reply to
josephkk

use the 0-5V signal to program a rail-to-rail PWM output. filter as neccessary.

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Reply to
Jasen Betts

but that has fixed gain. OP wants gain proportional to VCC.

Figure 3 has the current mirror drawn in a way I've not seen before.

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Reply to
Jasen Betts

that can take a voltage from 0 to 5V and output from around 0V(not critical) to near Vcc(but arbitrary) somewhat linearly.

decently but the issue is linearity and range.

- 5Rc/Re but we would like 0V or some low fixed voltage instead.

upper range(which is more crucial than the lower range) since Vin cannot swing down to 0V(I'm assuming the simple diode compensation scheme generally used).

upper range being more important. Vcc is somewhat arbitrary ==> May change after the design of the circuit ==> no component values can depend on Vcc to achieve specs. (Obviously one can assume that Vcc is within all the maximum voltage ratings of the components)

around 10-20C.

of the big problems).

the minimum Vcc-5Rc/Re not vary as well. If you get Vout,min=0V with Vcc=50V then Vout,min will be 450V for Vcc=50V.

and has nothing to do with the output voltage.

this is not the case.

specifically specified. In the real circuit it might be 234.32V but will not change in that circuit. What I do know is that it will be between 50V and 450V. If I can design a circuit that works over the whole range then I won't have to worry about changing resistors.

it to be between 50V and 450V. I would like to design the circuit so it behaves the same regardless without having to change resistors to make it so.

it's simple actually..

He wants a selectable gain control.

If the VCC is to change, then the gain control can be governed via the VCC level. Simple actually. A 50V minimum fixed reference against the VCC to generate a gain reference..

The 0..5V in the input to choice the level of that.

Yes, a simple OP-Amp circuit with a gain pot on the front end would work find. The 0..5Volts would simply be the range of the set gain..

And if there is not enough VCC on the rails to meet the output as required, then I guess one could even design in a fail safe error logic bit output :)

Oh well, sounds like something I can whip together in my sleep.. !

Jamie

Reply to
Jamie

that can take a voltage from 0 to 5V and output from around 0V(not critical) to near Vcc(but arbitrary) somewhat linearly.

decently but the issue is linearity and range.

upper range(which is more crucial than the lower range) since Vin cannot swing down to 0V(I'm assuming the simple diode compensation scheme generally used).

the upper range being more important. Vcc is somewhat arbitrary ==> May change after the design of the circuit ==> no component values can depend on Vcc to achieve specs. (Obviously one can assume that Vcc is within all the maximum voltage ratings of the components)

around 10-20C.

of the big problems).

the minimum Vcc-5Rc/Re not vary as well. If you get Vout,min=0V with Vcc=50V then Vout,min will be 450V for Vcc=50V.

and has nothing to do with the output voltage.

and this is not the case.

specifically specified. In the real circuit it might be 234.32V but will not change in that circuit. What I do know is that it will be between 50V and 450V. If I can design a circuit that works over the whole range then I won't have to worry about changing resistors.

it to be between 50V and 450V. I would like to design the circuit so it behaves the same regardless without having to change resistors to make it so.

Well he said he did not want to "change resistors" so I would take that to include not wanted to adjust a pot either!

Also he does not seemed to have mentioned the output current, a critical little detail at 500V...

--

John Devereux
Reply to
John Devereux

Elaborate on what it is that you find strange. It's a so-called "Norton" amplifier. ...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 | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

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| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Reply to
Jim Thompson

amplifier that can take a voltage from 0 to 5V and output from around =

0V(not critical) to near Vcc(but arbitrary) somewhat linearly.

work decently but the issue is linearity and range.

Rc/Re*Vin

Vout =3D Vcc - 5Rc/Re but we would like 0V or some low fixed voltage = instead.=20

our upper range(which is more crucial than the lower range) since Vin = cannot swing down to 0V(I'm assuming the simple diode compensation scheme= generally used).

components)/

with the upper range being more important. Vcc is somewhat arbitrary = =3D=3D> May change after the design of the circuit =3D=3D> no component = values can depend on Vcc to achieve specs. (Obviously one can assume that= Vcc is within all the maximum voltage ratings of the components)

only around 10-20C.

take care of the big problems).

could the minimum Vcc-5Rc/Re not vary as well. If you get Vout,min=3D0V = with Vcc=3D50V then Vout,min will be 450V for Vcc=3D50V.

voltage and has nothing to do with the output voltage.

operation and this is not the case.

specifically specified. In the real circuit it might be 234.32V but will = not change in that circuit. What I do know is that it will be between 50V= and 450V. If I can design a circuit that works over the whole range then= I won't have to worry about changing resistors.

change it to be between 50V and 450V. I would like to design the circuit= so it behaves the same regardless without having to change resistors to = make it so.

=20

Show us a circuit at both 50 V and 250 V.

?-/

Reply to
josephkk

Where I would have expected a diode connected transistor just a diode is drawn.

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Reply to
Jasen Betts

That's "artistic license" taken by the village idiots in the applications department. Unfortunately, quite early on, datasheets and appnotes weren't written by the designer. ...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  |             |
| Voice:(480)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.
Reply to
Jim Thompson

That's good to know, I spent an hour or so searching for a schematic with a proper current mirror in vain, trying to reason why a diode would be used there.

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Reply to
Jasen Betts

In the I/C world a "pure" diode, unencumbered by parasitic junctions does not exist. So you can rest assured that a "diode" on an I/C schematic is actually a three (or more) layer device, connected in diode mode.

That's why I'm always wary of more than trivial currents in ESD "diodes"... the positive rail one (in most processes) is actually a PNP with a very BIG collector (otherwise known as the whole device substrate)... and the "guard ring" diffusions to limit parasitic action DO NOT go all the way thru the die thickness. ...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  |             |
| Voice:(480)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.
Reply to
Jim Thompson

Check and double check. Start with something simple to illustrate: two "isolated" NPNs on a substrate. In one case, if properly done in a geometrical fashion, they will be matched extremely well (beta,leakage) with good thermal tracking (better than 10mSec hysteresis). In another case, in a high gain op-amp, the lateral PNP (base is the substrate) Q1 to Q2 could have a beta of 0.000001 or so and give hell in feedback - all the way across a chip where Q1 is part the op-amp input and Q2 is part of the output. A measly gain over 10^6 can be trouble in river city. Happened in the first cut for the uA741 at (the original) Fairchild. If i remember right, the solution was to wrap a vertical PNP around the output and make it a DCT to kill the beta across the chip.

Reply to
Robert Baer

[snip]

Almost correct. See...

formatting link

This is a typical bipolar process cross-section. CMOS? Still a P-type substrate.

A lateral-PNP is like a vertical-PNP except there are two P-base emitters side-by-side... plus a feeble attempt to kill the vertical device by adding buried layer

But you are right, all kinds of sneak paths to "brighten" (*) your day.

(*) I had a complex pin driver chip, ~1980, that glowed in the dark ;-) ...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  |             |
| Voice:(480)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.
Reply to
Jim Thompson

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