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Old 8th February 2008, 10:30 AM   #1
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Default Gain calculations

Hi

I'm just working through Douglas Selfs book and running through some of the calculations.

At the moment I'm looking at LF and HF gain

He gives LF gain to be gm*Beta*Rc

Could someone just clarify that the following assumptions are correct please?

gm in mA/V?

Beta is that of VAs transistor? and if a darlington stage is used, does that need to be used as beta*beta in the equation?

Rc is the output resistance of the VAS current source? I know it's going to be quite high, but can someone tell me how to calculate it for a common 2 transistor CC? I know I should be able to work this one out, but I think I've been looking at textbooks for too long and I can't see the wood for the trees now!

At the moment I'm getting a horrifyingly large number for a circuit setup with gm=95mA/V beta=14400 for 2 2SD669s in VAS, and 5Mohm for the VAS current source. I've done a lot of searching, but failed to find anything that can help yet.

Thanks for looking

Ian
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Old 8th February 2008, 10:39 AM   #2
AndrewT is offline AndrewT  Scotland
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2sd669 are good for drivers not for VAS and pre-drivers.
Use 2sa1360/c3423 or similar for their low capacitance.
If VAS power is low then To92 devices can be used.
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Old 8th February 2008, 10:50 AM   #3
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Thanks for the suggestion

I went with the 669s because Randy Slone uses them in almost all his designs, and they were recommended in a few threads on here. Self tends to use MJE340s quite a lot, but they really are slow! I do see what you mean about the cob for the 3423 - 5 times less than the 669!
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Old 13th February 2008, 05:57 PM   #4
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Hello,
Are there any other suitable Bjts for VAS other than 2sa/2sc?
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Old 13th February 2008, 09:47 PM   #5
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Look for something with a good ft, and low collector capacitance. Can no-one here help me out with the calculations? Or at least point me in the right direction.
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Old 13th February 2008, 10:01 PM   #6
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hello

Yes gm is in ma/V

I think it is the gain of a LTP where Rc is the collector load(hope i'm not wrong with this). Yes if you have a darl'ton you must multiply the two betas
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Old 13th February 2008, 10:17 PM   #7
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Brilliant, thanks for that. Very much appreciated

So we have (gain of LPT in mA/V)*(current gain of VAS)*(resistance of VAS collector load)

Which does make sense, and the first two I can calculate. The last piece of the puzzle here is how to work out or measure the effective resistance of the current source. Any takers on this one?
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Old 14th February 2008, 02:51 PM   #8
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For measuring the effective resistance of the current source, say you have a 15V supply. Put a low resistance in line with the current source so there is a voltage drop of 14V across the current source. No measure the current flowing into the collector of the current source. Lets say it is 1mA. Now put a larger resistor inline with the current source so the voltage across the current source is 1V. Now measure the current flowing into the current source. Lets say it measures to 0.991mA. Now do this formula: (14V-1V)/(1mA-0.991mA) which calculates to 1.44Mohms.

So it is basically change in voltage across current source/change in current flowing through current source which is R=V/I. The voltage change across the current source is created by changing the resistance inline with the current source which inturn changes the voltage across the current source. The current is measured at each different voltage level.

The effective resistance would always be changing but would stay high. In the example above, it should always be at least 1.44Mohms or higher in resistance as long as you are staying with in the limitations of the current source. This is a dynamic measurement. A static measurement would be voltage across current source/current flowing in current source which would be 14kohms for the first measurement, unless I am wrong.
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Old 14th February 2008, 03:31 PM   #9
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That's exaclty what I'm looking for, thank you very much

I'll get on with finding the output impedance of my CCS.

Thanks to everyone for their assistance
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Old 14th February 2008, 03:35 PM   #10
CBS240 is offline CBS240  United States
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Quote:
Originally posted by ian_elvar
Look for something with a good ft, and low collector capacitance. Can no-one here help me out with the calculations? Or at least point me in the right direction.

What Vce do you need? This is an important variable to consider when choosing VAS transistor(s).
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