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Old 19th October 2012, 03:23 PM   #351
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One way to get more gain is cascoding the BFs.
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Old 19th October 2012, 04:07 PM   #352
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Quote:
Originally Posted by Joachim Gerhard View Post
One way to get more gain is cascoding the BFs.
They are already cascoded.
Check out the schematic in attachment.
Attached Files
File Type: pdf Temp-TRANS-Z SIMPLIFIED.pdf (33.4 KB, 51 views)
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Old 19th October 2012, 04:19 PM   #353
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Ok, i see. You are loosing 6dB gain because there is no P-channel compliment.
The other way is using double value resistors and half value caps in the RIAA.
That gives you 6dB of gain.
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Old 19th October 2012, 04:45 PM   #354
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Quote:
Originally Posted by Joachim Gerhard View Post
Ok, i see. You are loosing 6dB gain because there is no P-channel compliment.
The other way is using double value resistors and half value caps in the RIAA.
That gives you 6dB of gain.
yes you are correct, no P-channel.
Is change value on the RIAA the only option? I had those custom values and change will completely change the set up.
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Old 19th October 2012, 05:09 PM   #355
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16 Bf in parallel is one way.
Raising the output impedance of the mirror is the other way.
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Old 19th October 2012, 06:02 PM   #356
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Quote:
Originally Posted by Joachim Gerhard View Post
16 Bf in parallel is one way.
Raising the output impedance of the mirror is the other way.
how am I going to raise the output impedance on the current mirror? Isn't that a function of rbe of the BJTs?

Simulation shows no increase on gain paralleling more ..it actually collapses (I think I loose control over the DC offset)
If anybody here wants to help with the simulation, please go ahead.
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Old 19th October 2012, 06:08 PM   #357
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Yes, more then 8 Bfs at the input brings trouble. To raise the output impedance of the mirror you can try to raise the value of the 120 Ohm emitter resistors. The other is "cascoding the cascode", so put an additional cascode at the output of the mirror.
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Old 19th October 2012, 06:51 PM   #358
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all that inner section of mirrors is not needed....there are simpler and better ways to shield the servo...

You can raise the resistors to 47 ohms each..and you can scale the mirror by using 220/110 or 220/47 ohms that will give you more current so you can get the gain you need by scaling the riaa components.
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Old 19th October 2012, 07:11 PM   #359
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Originally Posted by MiiB View Post
all that inner section of mirrors is not needed....there are simpler and better ways to shield the servo...

You can raise the resistors to 47 ohms each..and you can scale the mirror by using 220/110 or 220/47 ohms that will give you more current so you can get the gain you need by scaling the riaa components.
how would you shield the servo differently from this way?

Something I said I would have liked not to do is changing the RIAA values to fit the custom capacitors I have.
But it looks like there is no other way around to get more gain.
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Old 19th October 2012, 07:19 PM   #360
MiiB is offline MiiB  Denmark
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you could shield the current output by a jfet cascode... that will have substantial higer impedance than the mirrors

funney you want to save something so insignificant, and compromise in other vital areas... make the circuit using cheap parts to eval it...
if good and stable then you can go down the exotic route....

But I must say it again... in this kind of riaa the components matter a lot less.... and teflon caps are for many reasons vastly inferior to other industrial types of caps....

Last edited by MiiB; 19th October 2012 at 07:28 PM.
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