CRO Balanced Preamplifier

[Nick D]

Gday folks!
I was looking for some help on the net and came accross this forum, and am now a new member. Anyways, I'm studying engineering at uni and we have a project where we must build a preamplifier (balanced) to add high sensitivity ranges to a CRO. The input impedence must be >1k and must be able to drive a 100 Ohm load.

My plan is to cascade a 'cascode differential pair' (ie balanced input), with 20 dB gain, with a simple transistor combination to make up the final 20 dB (100 V/V is the requirement).

Now, I have the cascode pair running well, ie required bandwidth and gain, but I've run into trouble with my second stage. I've tried using a simple common emitter, but i can't seem to get it to give me 10V/V, even after applying the appropriate formula's.

I was wondering if there were any suggestions as to what type of second stage amp to use, whether a feedback pair or even something simpler. I wish to use only npn type transistors, preferrably BC548's as they supposedly work up to 300MHz (my required bandwidth is 40Hz to 15MHz).

Some assistance would be greatly appreciated

Nick D

 

[Analog]

Gday folks!
I was looking for some help on the net and came accross this forum, and am now a new member. Anyways, I'm studying engineering at uni and we have a project where we must build a preamplifier (balanced) to add high sensitivity ranges to a CRO. The input impedence must be >1k and must be able to drive a 100 Ohm load.

My plan is to cascade a 'cascode differential pair' (ie balanced input), with 20 dB gain, with a simple transistor combination to make up the final 20 dB (100 V/V is the requirement).

Now, I have the cascode pair running well, ie required bandwidth and gain, but I've run into trouble with my second stage. I've tried using a simple common emitter, but i can't seem to get it to give me 10V/V, even after applying the appropriate formula's.

I was wondering if there were any suggestions as to what type of second stage amp to use, whether a feedback pair or even something simpler. I wish to use only npn type transistors, preferrably BC548's as they supposedly work up to 300MHz (my required bandwidth is 40Hz to 15MHz).

Some assistance would be greatly appreciated

Nick D

For a Cathode Ray O'scope (CRO), I can understand why you would need this in some apps. But using a CE amp won't work with the bandwidth you desire, due to the Miller effect. A second cascode stage followed by an emitter follower to give you a low output impedance may be the ticket.

 

[Nick D]

CRO Preamp

[/QUOTE]Thanks dude ,

I was toying with the idea of using two cascodes, but wouldn't I be able to just use one and amp it up to a gain of 100, followed by the emitter follower?

I've been told by a fellow student that it's better to have most of the gain in the first stage, so that you can sacrifice the gain of the latter in order to increase bandwidth. Is this true?

Also, is there a way of nesting one cascode inside the other, to avoid a bulky circuit? I've noticed a similar style when looking at the internals of a 741 (ie the differential input stage seems to be nested in another differential or current mirror stage). I've tried to mimic portions of it but get a bit lost.

regards
Nick D

P.S This post is particularly directed to Analog, (or anyone else interested) who replied to my original.

 

[Roff]

Nick, What is the maximum amplitude of your input signal?
Are you required to use discrete transistors? Op amps would be simpler, and provide superior performance. Also, have a look at the schematic of ua733. It's obsolete, but I used it many years ago, and it's a simple circuit.
Can you post a schematic of what you have now?

 

[Analog]

I was toying with the idea of using two cascodes, but wouldn't I be able to just use one and amp it up to a gain of 100, followed by the emitter follower?

I've been told by a fellow student that it's better to have most of the gain in the first stage, so that you can sacrifice the gain of the latter in order to increase bandwidth. Is this true?

Also, is there a way of nesting one cascode inside the other, to avoid a bulky circuit? I've noticed a similar style when looking at the internals of a 741 (ie the differential input stage seems to be nested in another differential or current mirror stage). I've tried to mimic portions of it but get a bit lost.

regards
Nick D

P.S This post is particularly directed to Analog, (or anyone else interested) who replied to my original.

Yes, you want the most gain in the 1st stage, but I usually attribute this to reducing the overall noise in a circuit. The 1st stage is critical in terms of getting the lowest noise possible, so that subsequent stages don't just amplify the noise generated in the 1st stage.

 

[Roff]

Analog, can you edit your last post to fix the quotes? It confused the hell out of me!

 

[Analog]

Analog, can you edit your last post to fix the quotes? It confused the hell out of me!

Not sure how that happened... Fixed.

 

[Roff]

Not sure how that happened... Fixed.

Thanks a lot!

 

[Nick D]

att: Ron H and Analog

My apologies gentlemen,
I'm not usually available over the weekend because I work in a bar.

In regards to the schematic, I only have a model of the cascode pair, and have been playing around with different feedback pairs, but I'de like to avoid using pnp's. I'm thinking of going ahead with the double cascode arrangement, say 15 gain in the first stage and around 6.5 - 6.8 in the second. Does this sound appropriate in terms of noise and what not? Anyways, I'll post a schematic of what I have in the next 24 hrs.

Cheers
Nick

P.S. I must be able to produce an output of 2 Vpp, ie 20 mVpp input. There is no max or min requirement. And yes - all discrete. I'm not allowed to use any IC's apart from voltage regulators.

 

[Nick D]

Cicuit diagram to CRO Preamp

Ron,

I've posted a schematic of what i intend to use. After adding the second cascode stage, i seem to have lost my gain in the first. I'm using Multisim7 to simulate. The collector resistances in the first were originally 175 Ohm, but i have been playin around with the values a bit. I intend on adding a final emmiter follower stage to increase the output current. Am i on the right track?

Thanx
Nick

P.S My input signal is 20mVpp but the shown amplitude is in RMS. Also, my output is node 11

 

[Nigel Goodwin]

You'd do better to post your picture as a GIF or PNG, it will be much smaller and better quality than a JPG - which is too big and barely legible.

From what I can see your resistor values all look exceedingly low though?.

 

[Nick D]

Thumbnails

Thanx for the input Nigel.

With respect to the thumbnail, when I previewed it, I found that by expanding it (ie the enlargement tab on the bottom right of the screen) the circuit becomes as bright as day. Could this just be on my end?

Regarding the resistances, I'm designing for a 10mA collector current. My lecturer, in the early stages of the task, mentioned keeping the values lower. With my supply rails being only + and - 4.5V aren't those resistors typical for such a collector current?

Cheers
Nick

 

[Nigel Goodwin]

Thanx for the input Nigel.

With respect to the thumbnail, when I previewed it, I found that by expanding it (ie the enlargement tab on the bottom right of the screen) the circuit becomes as bright as day. Could this just be on my end?

By using GIF or PNG the picture is perfect, and the file size is far smaller than JPG - you should only use JPG for photos, which is what it's desinged for.

Regarding the resistances, I'm designing for a 10mA collector current. My lecturer, in the early stages of the task, mentioned keeping the values lower. With my supply rails being only + and - 4.5V aren't those resistors typical for such a collector current?

The picture was too blurred to make much out, but is there a specific reason for running at 10mA?, which seems rather high for preamps?.

 

[Nick D]

Ok Understood, thanx.

I need to pump 2 Vpp into 100 Ohms, around 40mW. This is one of the requirements. It's a fairly pointless task, just designed to explore the characteristics of amplifiers in general. Do you still think 10mA is excessive?

I also need to make a few adjustments to the values in the cct (abrev. for 'circuit' in australia) such as gain in both stages. I wish to have 20V/V in the first stage and 5V/V in the second.

Nick

P.S Should I repost the thumbnail?