Operational Amplifier design help

Thread Starter

elrhazouani

Joined Feb 8, 2011
15
I want to design an op amp with the following specifications:

  • Allowed transistors are the 3904 (npn BJT), 3906 (pnp BJT), 2N3819 (n-channel JFET).
  • The input is a 1 mV amplitude source at 1.0 kHz riding on a DC value between -1.0V and +1.0V. The source has a 4 kW source resistance.
  • The primary goal is to provide a 2 V amplitude sinusoidal output across a 100 W load with an average value as close to 0.0V as possible.
  • No capacitors are allowed.
  • Standard resistor values must be used.
  • Supply voltage is to be provided by no more than 2(two) 9V batteries.
 

Adjuster

Joined Dec 26, 2010
2,148
The most challenging aspect of this would seem to be the need to obtain an output with zero DC content without using capacitors, given an input with such a wide range of possible DC values.

Do you know if any other components than transistors and resistors would be permitted?
 

Georacer

Joined Nov 25, 2009
5,182
Is this a homework question? Are you required to follow any conventions or other simplifications?

OpAmp datasheets often have transistor schematics included.
 

T.Jackson

Joined Nov 22, 2011
328
The most challenging aspect of this would seem to be the need to obtain an output with zero DC content without using capacitors, given an input with such a wide range of possible DC values.
ALL capacitors block DC but pass (n) degree of AC depending on what is called their 'capacitive reactance' xc = 1/ 2pi (fc)
 

Georacer

Joined Nov 25, 2009
5,182
T.Jackson said:
We might take you 2% serious if you can answer this:

Requirements:

  • 6 resistors ...
  • 5 specific Vrefs of: 1, 2.2, 3, 3.3, and 4.5VDC
  • IT = 20mA
  • Calculate value of R1 - R6
[...]

Show all workings, natural voltages measurable from ground.
@T.Jackson

No one in this forum has the authority to put members' abilities to the test and act like some self-proclaimed examiner for E.E examinations. Each member has the responsibility of his words. What others think of him is their business. You are not to say who has the right of speech in this forum. Moderators and Administrators do.

Furthermore, you are leading this thread offtopic with provocative comments.

You are requested to stop any provocative and disruptive action in this site. This is your last warning before a ban of some time period takes effect.
 
Last edited:

Thread Starter

elrhazouani

Joined Feb 8, 2011
15
this is the kind of amplifier I was referring to but the gain is too low around 50, and I need 2000 v/v gain:

any suggestions will be much appreciated !
 

Wendy

Joined Mar 24, 2008
23,415
That link does not work. You need to attach images instead, local hosting (which AAC offers) is always preferred.

Click the <Go Advanced> button below your post,

Then click <Manage Attachments>

This will allow your to upload your image.

.png or .gif formats preferred, as .jpg will fuzz an image badly (it is meant for photographs, not schematics).
 

SgtWookie

Joined Jul 17, 2007
22,230
Here is the 1st image:


It is now hosted on AAC.

[eta]

Here is the 2nd schematic:


They are now both hosted on AAC.
 
Last edited:

thatoneguy

Joined Feb 19, 2009
6,359
I want to design an op amp with the following specifications:

  • Allowed transistors are the 3904 (npn BJT), 3906 (pnp BJT), 2N3819 (n-channel JFET).
  • The input is a 1 mV amplitude source at 1.0 kHz riding on a DC value between -1.0V and +1.0V. The source has a 4 kW source resistance.
  • The primary goal is to provide a 2 V amplitude sinusoidal output across a 100 W load with an average value as close to 0.0V as possible.
  • No capacitors are allowed.
  • Standard resistor values must be used.
  • Supply voltage is to be provided by no more than 2(two) 9V batteries.
I'm guessing the 'W' in the requirements are Ω due to context.

Is there a limit on number of each type of transistor is used?

The 2V amplitude output required, is that RMS or peak to peak?

A differential amp is all I come up with for the variations in input bias without capacitors. Gain of 200 at 1kHz only? Guess, the exact frequency doesn't matter as everything must be DC coupled anyway.
 

thatoneguy

Joined Feb 19, 2009
6,359
I know, it was for me as well.

I used the URL from the 2nd image combined with the filename from the bad link to get the schematic image.
Quite Clever. That's how people find all the "private" photos on an account, another reason not to name them logically.

However, I am still impressed at that prowess, most people over 40 don't know that trick.
 

thatoneguy

Joined Feb 19, 2009
6,359
Use a Sziklai/Compound pair Long Tailed Pair for input. Similar to the diagram at section 7 of ESP Sziklai Page

That should give you the gain you need for a very low gain output stage.

--ETA: For 0.0V output, you'll need to use a Class B or Class AB output amp.
 
Last edited:

thatoneguy

Joined Feb 19, 2009
6,359
What was the behavior when it didn't work?

Can you attach images of the schematics you have tried and haven't worked, preferably with output waveforms so what didn't work is apparent?

Attach by clicking on the paper clip in the advanced editor, it allows you to upload images for free hosting on this server, so they are available if a hosting site exceeds bandwidth or whatnot.
 
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