JFET cascoded with tube gain calculation
 User Name Stay logged in? Password
 Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read Search

 Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

 Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Ken Newton
diyAudio Member

Join Date: Apr 2003
Location: Eastern Pennsylvania
Quote:
 Originally Posted by dia thien thai I guess nobody in this forum knows exactly how to calculate the gain or to analyze the circuit. People here normally judge a circuit by ear. Except me, of course.
I'll give it try. The JFET functions as a signal modulated current source. The scale of modulated current is determined by the effective transconductance, 'gm' (Amps per Volt) of the JFET stage, which includes the source resistor. This can be found by taking the datasheet transconductance figure, usually shown as milliamps per Volt, and converting milliamps per Volt in to Amps per Volt. Then, find the reciprocal value (1 / gm), then sum in the source resistor value (R2). Finally, find the reciprocal of this sum. The result will be the net effective transconductance of the JFET stage, in Amps per Volt.

Since the JFET drain has a high impedance relative to the tube's cathode, the current conducted through that cathode is essentially contolled entirely by the JFET. Since the tube's cathode, anode (plate), and the anode (plate load) resistor are in series, the current conducted through the cathode will also be conducted through the plate load resistor, creating the output voltage.

Let's take an example, just for illustration. If the datasheet gm of the JFET were, let's say, 200mA per Volt and the source resistor (R2) were 20 ohms, the effective transconductance would be: (1 / 0.2A per Volt) + 20 ohms = 5 + 20 = 25, then 1 / 25 = 0.04A per Volt.

So, a 1 Volt input sgnal would produce a 0.04A current through the JFET drain. Which therefore means there would be a 0.04A current through the tube's plate load resistor. Now, we simply apply ohms law to determine the voltage amplitude across the plate resistor: 0.04A x 25kohms = 1,000 Volts! Of course, we don't need 1,000 volts, but it gives us the input to output voltage ratio (the gain) of the circuit for use with small signals. If 1 Volt in would give 1,000 Volts out, then the voltage gain is 1000x, which is 60dB.

Remember, this is just an first order calculation. I should expect that there might be second order effects that could somewhat reduce the calculated gain, which might be why Allen Wright (rest in peace) said to build the circuit and measure the actual gain.

Last edited by Ken Newton; 30th May 2012 at 02:11 PM.

Osvaldo de Banfield
diyAudio Member

Join Date: Dec 2011
Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina
Quote:
 Originally Posted by Gordy Because people often have limited time, and therfore can not afford to build on the off-chance that it might be right. Because people often have limited money, and therfore can not afford to buy a stock of components just to experiment. Because people often have limited time and money, and therfore can not afford to have circuits explode because they were designed wrong. Because calculating is a proven professional way of taking a step towards a successful design. (And I don't just mean in audio, but in any technical subject). Because calculating goes hand-in-hand with understanding, and understanding often leads to improved design, easier modification, easier fault-finding, and a good chance of success, and hence ultimately leads to a sense of intellectual reward. If you want to do it the other way then best of luck to you, however I would encourage folks to understand and calculate before 'buying and trying'.
I strongly agree with this post.
__________________
Osvaldo F. Zappacosta. Electronic Engineer UTN FRA from 2001.
Argentine Ham Radio LW1DSE since 1987.

SY
On Hiatus

Join Date: Oct 2002
Location: Chicagoland
Quote:
 Originally Posted by Ken Newton Let's take an example, just for illustration. If the datasheet gm of the JFET were, let's say, 200mA per Volt...
100% agree, Ken. It should be noted that real FETs of the same nominal type are likely to have gms varying by 3:1 or more. Thus Allen's advice about building and measuring, unless you're willing to overbuy FETs and sort them to get the gm you want.
__________________
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."

 30th May 2012, 02:26 PM #14 DF96   diyAudio Member   Join Date: May 2007 The gain of a cascode can be looked up in most good textbooks (which may be why there was reluctance to answer the question). The first order calculation has been given in this thread: gm x load. Second order corrections bring in the gm of the upper valve and the mu of the lower one, but these are small changes and can usually be ignored as they are within the usual range of spread of device characteristics. My view is that if you can't at least estimate the gain, frequency response etc. of your circuit then you are not designing, merely throwing parts together. Simulation is a good way to check a design, but a poor way to do design. Measurement is a good way to check a design and should then lead to debugging: if gain is 30% lower than you expected then you may need to find out why.
 4th June 2012, 11:10 AM #15 andyjevans   diyAudio Member   Join Date: Sep 2003 Location: London A bit off topic, but could this FET-tube cascode be used as an output from a differential DAC? I have a RAKK DAC with outputs of .4V per leg with .4VDC on each leg. They usually use a couple of 91 ohm i/v resistors. Active output
Ken Newton
diyAudio Member

Join Date: Apr 2003
Location: Eastern Pennsylvania
Quote:
 Originally Posted by andyjevans A bit off topic, but could this FET-tube cascode be used as an output from a differential DAC? I have a RAKK DAC with outputs of .4V per leg with .4VDC on each leg. They usually use a couple of 91 ohm i/v resistors. Active output
Hi Andy,

I should think that jfet/tube cascode would have far too much gain. It believe it is intended as the input stage of a phono amplification chain. If you intend to take a differential output (both 'legs') from your DAC, then you will have a 0.8V output signal, which would require only 2.5x amplification to attain the ubiquitous 2V DAC output level. What is the full scale signal amplitude which you want to achieve?

Last edited by Ken Newton; 4th June 2012 at 12:14 PM.

 4th June 2012, 12:16 PM #17 SY   On Hiatus     Join Date: Oct 2002 Location: Chicagoland It depends. If you have a current output DAC terminating in small resistors, a FET-tube cascode can work very well. If you have a voltage output DAC, the gain is likely to be too high (as Ken says) and there's not much advantage to using a complex circuit with low PSR. edit: Looking at your link, that DAC has a current output which is meant to terminate into a virtual ground. I don't know if using 91R will degrade the performance- you might ask Dave Davenport directly, he's a very sharp guy. If the 91R is OK, you need very little gain out of the following stages, maybe between 0 and 6 dB; it merely needs buffering. Cascode is not the first choice for that. __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
 4th June 2012, 12:47 PM #18 Ken Newton   diyAudio Member     Join Date: Apr 2003 Location: Eastern Pennsylvania SY, I believe the RAKK DAC utilizes the PCM1794A, which is one of the few current output DACs which performs well while developing an substantial A.C. voltage on it's output pins - meaning, with passive resistor I/V. My own experimental DAC design utilizes this chip with 150 ohm passive I/V resistors, wherein THD measures around 0.002% @ -3dBFS via my 24-bit soundcard and FFT audio analyzer software. I believe that several commercial digital source vendors, such as Audio Research and Ayre, utilize the PCM1794A with passive resistor I/V values of up to 200 ohms. Passive resistor I/V has been subjectively far more transparent sounding than the op-amp based transimpedance converter I had once used. Last edited by Ken Newton; 4th June 2012 at 01:03 PM.
 4th June 2012, 12:50 PM #19 SY   On Hiatus     Join Date: Oct 2002 Location: Chicagoland If that's the case, a simple buffer ought to do fine. If one wants pro levels, 6dB of gain will get you there. How do you work the anti-imaging filter? __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
Ken Newton
diyAudio Member

Join Date: Apr 2003
Location: Eastern Pennsylvania
Quote:
 Originally Posted by SY If that's the case, a simple buffer ought to do fine. If one wants pro levels, 6dB of gain will get you there.
I've A.C. coupled a single phase of my DAC output to my JFET line-stage with no intervening active circuitry. The 400mVRMS DAC output is plenty, given the 20dB gain of the line-stage.

Quote:
 How do you work the anti-imaging filter?
I use a first order passive filter, a 100pF NP0 ceramic SMD across the PCM1794A's differential current outputs, as near as I can get to the chip's pins. I've not yet experimented with multi-pole passive filters, as they would require utilizing either inductors or cascaded lossy RC stages. But who knows, my DAC's output spectra may be provoking audible IMD products in my line-stage or power amp, which would be reduced with a higher order filter. Another experiment to add to the list I suppose.

Last edited by Ken Newton; 4th June 2012 at 01:41 PM.

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Site     Site Announcements     Forum Problems Amplifiers     Solid State     Pass Labs     Tubes / Valves     Chip Amps     Class D     Power Supplies     Headphone Systems Source & Line     Analogue Source     Analog Line Level     Digital Source     Digital Line Level     PC Based Loudspeakers     Multi-Way     Full Range     Subwoofers     Planars & Exotics Live Sound     PA Systems     Instruments and Amps Design & Build     Parts     Equipment & Tools     Construction Tips     Software Tools General Interest     Car Audio     diyAudio.com Articles     Music     Everything Else Member Areas     Introductions     The Lounge     Clubs & Events     In Memoriam The Moving Image Commercial Sector     Swap Meet     Group Buys     The diyAudio Store     Vendor Forums         Vendor's Bazaar         Sonic Craft         Apex Jr         Audio Sector         Acoustic Fun         Chipamp         DIY HiFi Supply         Elekit         Elektor         Mains Cables R Us         Parts Connexion         Planet 10 hifi         Quanghao Audio Design         Siliconray Online Electronics Store         Tubelab     Manufacturers         AKSA         Audio Poutine         Musicaltech         Aussie Amplifiers         CSS         exaDevices         Feastrex         GedLee         Head 'n' HiFi - Walter         Heatsink USA         miniDSP         SITO Audio         Twin Audio         Twisted Pear         Wild Burro Audio

 Similar Threads Thread Thread Starter Forum Replies Last Post atiq19 Tubes / Valves 11 22nd November 2009 12:55 PM rbroer Solid State 22 1st July 2008 02:43 PM bobsinclar Pass Labs 26 21st September 2007 08:23 PM jh6you Pass Labs 60 17th March 2006 01:19 AM Magura Pass Labs 44 31st January 2006 09:00 PM

 New To Site? Need Help?

All times are GMT. The time now is 11:35 AM.