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Old 30th September 2012, 06:51 PM   #1
kisshu is offline kisshu  United States
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Default unusual amplifier topology jfet

I'm trying to understand this unusual amplifier topology from the 1980's
revox b285 receiver. It has a jfet differential pair with bjt cascode but i don't understand the circuit above or the vas section; also there is an unusual use of an ic dual opamp LF353 in the global feedback circuit. would anyone like to help descibe the topologies starting with the IPS?

there maybe improvements to make to the circuits as well.

thd is listed as <.03% 20-20k
<.005% at 1khz 70 watts into 8ohms
dynamic headroom 2.5db 8ohms
rise time 4us 8ohms -( not sure how it relates to slew rate?)

Its an interesting topology to analyze and learn about.
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Old 30th September 2012, 07:56 PM   #2
Bensen is offline Bensen  Belgium
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Hi there,

If I understand you correctly, you are speaking of the two upper BJT's where the Base's are connected to eachother.
==> do a search for "current mirror".
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Old 30th September 2012, 08:13 PM   #3
kisshu is offline kisshu  United States
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it looks like a current mirror but the way its connected looks strange booth collectors are connected to the cascode collectors it looks like a quad current mirror? how does it work with that 100k R? is it stabalizing the cascode?
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Old 30th September 2012, 08:17 PM   #4
kisshu is offline kisshu  United States
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also there is a 2 circled that appears in various places in the entire schematic anyone know what that would refer to?
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Old 1st October 2012, 06:16 PM   #5
alex mm is offline alex mm  Europe
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.......this components marked 2 in circle ,are composed by 2 pices , C100 it's
5.6pF aded with 22 pF , see BOM list , and you will descover
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Old 1st October 2012, 06:57 PM   #6
knutn is offline knutn  Norway
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Quote:
Originally Posted by kisshu View Post
it looks like a current mirror but the way its connected looks strange booth collectors are connected to the cascode collectors it looks like a quad current mirror? how does it work with that 100k R? is it stabalizing the cascode?
It's a normal current mirror, but the 100k resistor is lowering the open loop gain (and increasing the bandwidth) of the input stage.
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Old 1st October 2012, 07:11 PM   #7
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by kisshu View Post
also there is an unusual use of an ic dual opamp LF353 in the global feedback circuit.
Hi,

Its probably a DC Servo to control the outputs DC offset.

rgds, sreten.
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Old 2nd October 2012, 12:31 AM   #8
davada is offline davada  Canada
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Quote:
Originally Posted by kisshu View Post
I'm trying to understand this unusual amplifier topology from the 1980's
revox b285 receiver. It has a jfet differential pair with bjt cascode but i don't understand the circuit above or the vas section; also there is an unusual use of an ic dual opamp LF353 in the global feedback circuit. would anyone like to help descibe the topologies starting with the IPS?

there maybe improvements to make to the circuits as well.

thd is listed as <.03% 20-20k
<.005% at 1khz 70 watts into 8ohms
dynamic headroom 2.5db 8ohms
rise time 4us 8ohms -( not sure how it relates to slew rate?)

Its an interesting topology to analyze and learn about.
Hi kissue,

The circuit above is a current mirror. It increases the gain of the input stage by raising the impedance of the load on the cascode transistor collectors. The current drawn in the inverting leg, with respect to the inputs, is mirrored in the non inverting leg of the current mirror. Of course the signal itself is inverted. The VAS is an ordinary one with a buffer transistor emitter coupled to the VAS transistor. This increases the gain and bandwidth of the VAS and isolates the input stage by increasing the impedance, again reducing the load on the IPS. Together this topology can increase the gain as much as 1000 times or more. The diode D403 is just a clamp to help prevent sticking if the amplifier is driven to clipping. The VAS is coupled in a folded cascode to the next stage. It's a two stage VAS.
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Last edited by davada; 2nd October 2012 at 12:50 AM.
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Old 2nd October 2012, 12:55 AM   #9
CBS240 is offline CBS240  United States
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Quote:
Originally Posted by kisshu View Post
; also there is an unusual use of an ic dual opamp LF353 in the global feedback circuit.
It appears to be an integrator for DC servo control.

Not to belittle an existing design but there are a couple of things I would have done differently. A Wilson mirror in the IPS would keep Vce and thus Pd of the mirror transistors equal. Or, I would have used a cascode arrangement, i e common base instead of a Zener for the buffer that feeds the folded cascode VAS. This way you could use a low Vce small signal device that has a large current gain, like a 2N5087 or a even a J-fet and eliminate a stage within the global loop. But then this is why I make my own amps.
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Old 2nd October 2012, 05:55 PM   #10
kisshu is offline kisshu  United States
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Quote:
Originally Posted by sreten View Post
Hi,

Its probably a DC Servo to control the outputs DC offset.

rgds, sreten.
Isn't it involved in the error correction? IC LF353 looks like its inverting the signal -some sort of high impedance inverting buffer that feeds back to the jfet differential pair? also the el capacitor appears to cause a filter effect. would like to know how it works.
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