Leach opamp

[udip]

Hi.



I was searching for a discrete opamp designs and saw an old
article by W.Marshall Leach about high bandwidth preamp.
In the article he has two designs of a phono preamp and an
output stage. The designs are actually discrete opamps, one
using BJT's at input and the other FET's.


The opamp is designed to work with +-24v.
I ran simulations of the opamp and everything seems ok,
but for use with +-15v it's a bit weak.
Maybe I should add a current source for the LTP, any Ideas?


Help is appreciated...
Thanks,

Udi.

Edit: I used bc550/560 instead of the original 2n5210/5087


The schematics:

[udip]

Well, I replaced the tail resistor with current source....

I'v read that for balanced operation of the input transistors,
R1 should equal 2 times R2, and if not distortion will rise
Is this true?


Thanks,


Udi.

[AndrewT]

Hi,
no or yes?
in mk2 the tail current through the ccs forces 1mA through each leg of the ltp.
the 3k6 generates 3v6 and forces 3v across the second LTP tail resistor which in turn creates the bias on the output all ok and r1 is not tied to r2.
In mk1, r1 &r2 both dissipate about half supply voltage. r1 carries twice the current of r2 and for a balanced LTP the relationship is important. So for resistor loading the times 2 relationship is yes.
regards Andrew T.
edit I hope I have the I's & R's the right way round. I cannot see the schematic while typing this reply.

[AndrewT]

Hi,
why the different values of second LTP load 1k8 & 1k6?
What is the LED voltage? 470r might be a little low for 2mA.
regards Andrew T.

[Bricolo]

You can also read Mr Pass's article about discrete opamps, very good information too

[udip]

Originally posted by AndrewT
Hi,
why the different values of second LTP load 1k8 & 1k6?
What is the LED voltage? 470r might be a little low for 2mA.
regards Andrew T.

Hi Andrew,

The different values are like in the original schematic.
when I ran simulation, I found that if both values are equal
than the transistors are not balanced.... though I don't
know why..it seems strange...

the led is 3mm and it's voltage - 1.65, so
I=(1.65-0.65)/470=2.12ma, so that's ok.


Thanks...

Udi.

[fab]

hi udip,

When using jfet like 2 independant 2N5457 ensure that they are matched because in a real life circuit you will encounter problems if their characteristics (IDSS, yfs, VGSoff, etc) are too different (DC offset, linearisation, etc). On way to compensate is the use of a trim pot between the sources of the jfet.

fab

[udip]

Hi fab,


I must say that you are correct, I'v just finished building this opamp with no special matching between the fets (just idss), and it worked, but I had something like 5 - 10v offset!!

I guess it's important to match them perfectly...

Anyone knows how to cascode the input fets??



Thanks,

Udi.

[udip]

Hi again!!



Day by day I'm learning new things about opamp design,
input stage, vas, output, current sources, current mirrors, etc.

But why do we need a current source (T5) for a current source
like in this elektor scheamtics??!?! I'm confused...



Thanks,

Udi.

[Mr Evil]

Originally posted by udip
...But why do we need a current source (T5) for a current source
like in this elektor scheamtics??!?!

The current sources rely on the voltage across the LEDs in order to provide a constant current. Although LEDs nominally have a constant voltage drop across them, it actually varies slightly with current, so if you used e.g. a resistor in place of T5, variations in supply voltage would yield (small) variations in current out of the current sources. So T5 increases PSRR.