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Old 17th July 2012, 11:01 AM   #21
owdeo is offline owdeo  Australia
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You can actually just copy and paste the text for each additional model into the relevant library file (eg bipolars into "standard.bjt") in the LTC directory at:

C:\Program Files\LTC\LTspiceIV\lib\cmp

That way you don't need the .include statement (ie I'm lazy ). Bob Cordell's models appear with a C suffix so are easily identified when selecting devices.
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Old 17th July 2012, 11:14 AM   #22
owdeo is offline owdeo  Australia
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BTW, why are you running so much current through the VAS? I would think it only needs say 6mA or less, certainly not 40 odd. You're probably just dissipating power for no good reason. Have a look at the currents around the VAS when outputing a 50kHz square wave (as a worst case scenario) and you'll probably find you need much less to slew Cdom adequately. Or perhaps some of the more experienced designers can suggest the best method.
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Old 17th July 2012, 11:58 AM   #23
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Here's something similar using different parts. The LTP drives an emitter follower which in turn drives an SRPP and a final emitter follower. I don't think there's anything exotic about it. DC offset on the output is -5mV. S/N is 125dB @ 1kHz. Distortion is 6x10^-5% or so for 1.4Vrms output into 10k ohm load. Bandwidth and phase is flat to a GHz or so, then there's a bump. I should probably fine tune the feedback network a little, but I just thought I'd toss another idea out there. The diodes are the CRD (CLD) constant current type. The trim pot is for adjusting the balance of the SRPP.
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Old 17th July 2012, 12:05 PM   #24
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by dirkwright View Post
Bandwidth and phase is flat to a GHz or so, then there's a bump.
Are you sure ?
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Old 17th July 2012, 12:08 PM   #25
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Quote:
Originally Posted by Mooly View Post
Are you sure ?
All I know is what the software is telling me. Of course, there's no RF filter on the input...

I guess maybe I misspoke. It's early in the morning here. Obviously, it's not flat to a GHz. I meant that there was a bump AT 1GHz or so. sorry.
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Last edited by dirkwright; 17th July 2012 at 12:10 PM.
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Old 17th July 2012, 12:17 PM   #26
Mooly is online now Mooly  United Kingdom
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An amplifier of that topology and component values won't work at those frequencies.

I don't know why your simulation program shows such a peak in response at hf. For one thing all the semiconductors are low frequency types (in the scheme of things). 1ghz bandwidth would make a good aerial amp... it's even considered part of the microwave band.
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Old 17th July 2012, 12:18 PM   #27
Calvin is online now Calvin  Germany
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Hi,

Oli, first memorize Vbe to be roughly 600mV (NPN) or -600mV (PNP). Ony could say that the Emitter of an NPN follows its Base voltage 600mV lower and that the Emitter of a PNP follows its Base voltage 600mV higher.
Then look at Your last schematic again.
The Base of Q6 sits 600mV below the emitter at ~14.4V.
Since Q6 needs only tiny base currents there´s nearly no voltage drop over R6 (its function is to prevent oscillations).
So the node R5/R6/E(Q5) also sits 600mV below the supply voltage at 14.4V. These 600mV divided by the resistance value of R5 defines the current through R5 and into the Emitter of Q5 (base current into Q6 may be neglected here), giving ~6mA.
Now the base of Q5 sits 600mV below its Emitter, hence at 2xVbe below the supply voltage at 13.8V.
Neglecting the low base current of Q7 (hence neglecting voltage drop over R9) the base of Q7 sits at 13.8V and as such its Emitter at 14.4V.
So there´s a voltage drop of 600mV (1xVbe) over R13. With just 10Ohms this calculates to 60mA....way too high. R13 could better be chosen to 100-150Ohms (4-6mA).
Now omit with R2....its of no use here.
The current through R13 and Q7 also flows through D1 and D2, which generates a voltage drop of 2x600mV. The base-emitter path in a transistor functions like a diode, hence the similar voltage drops. So Q8 as well as Q11 open and current flows, but not very defined. If You add a 3rd diode and two Emitter resistors Re8 for Q8 and Re11 for Q11 You can easily calculate the resultant current through Q8 and Q11. Its the third diodes voltage drop divided by the sum of the two resistors. With Re=27Ohms You should end up around 10-15mA. The Diodes need to be thermally coupled with Q8 and Q11. Still though the working point might not be stable over temperature. In that case a ´transistorized Diode´ might be required.
Attached is a ciruit that has been published by Elektor in 1994, the discrete Ulti-Preamp. (I replaced the expensive matched MAT02/MAT03 input transistors with the usual BC550/560)
discrete Ulti-Preamp.asc
In this schematic the Diodes are replaced by a ´transistorized Diode´ here composed of two transistors (Q9 thermally coupled to Q11, Q10 to Q12), two resistors R18/19 and a linear Poti P1. (Q11/Q12 could be the BD139/140 Pair instead)

jauu
Calvin
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Old 17th July 2012, 12:36 PM   #28
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Quote:
Originally Posted by Mooly View Post
An amplifier of that topology and component values won't work at those frequencies.

I don't know why your simulation program shows such a peak in response at hf. For one thing all the semiconductors are low frequency types (in the scheme of things). 1ghz bandwidth would make a good aerial amp... it's even considered part of the microwave band.
Yeah, I agree, so that's why I'm not worried about it.
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Old 17th July 2012, 12:39 PM   #29
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Quote:
Originally Posted by Calvin View Post
Hi,


Attached is a ciruit that has been published by Elektor in 1994, the discrete Ulti-Preamp. (I replaced the expensive matched MAT02/MAT03 input transistors with the usual BC550/560)
Attachment 292030
In this schematic the Diodes are replaced by a ´transistorized Diode´ here composed of two transistors (Q9 thermally coupled to Q11, Q10 to Q12), two resistors R18/19 and a linear Poti P1. (Q11/Q12 could be the BD139/140 Pair instead)

jauu
Calvin
That's a very sophisticated circuit! (well, to me anyway) The trim pot didn't show up in my LTSpice. Oh well.
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Old 17th July 2012, 04:10 PM   #30
Calvin is online now Calvin  Germany
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Hi,

well just get rid of Q1,2,6 and throw the damn never really good working dc-servo away, put a decent 1000µ cap between feedback-R30 and gnd and it looks like Olis circuit.
Replace the pot by a fixed resistor value of 25k-27k to vary the output bias current. It is connected as variable resistance anyway.
Oh, btw. if You haven´t recognized it already.....just dragging the "in" label from signal source Vsine to Vsquare changes the sim from sinewave to squarewave. .param Freq=xx changes signal frequency, .param Vin=xx changes signal amplitude.

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