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 Positive-feedback tech - the 'Bootstrap'
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 Return to top of page · Post #: 1 · Written at 1:27:27 AM on 25 September 2020.
NewVista's avatar
 Location: MilwWI, US
 Member since 10 May 2013
 Member #: 1340
 Postcount: 800

In another post here I touched on an example of positive feedback in cochlear hearing transduction. Another example of positive feedback (in electronics) would be the 'bootstrap' circuit: Here's what my trusty 1964 GE Transistor Manual has to say on the subject:

"..Another example of positive feedback with less than unity gain is the bootstrapping of biasing resistors in a transistor amplifier. This is done to minimise the shunting effect these resistors have on the input impedance of the circuit.." (page 216)


 
 Return to top of page · Post #: 2 · Written at 8:29:01 AM on 25 September 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 2018

Yes bootstrapping was commonly used in early SS "stereogram" circuits for that purpose.

I recall a staff tech training session in 1969 where I had to explain bootstrapping to the other service techs, transitioning from "tubes" to transistors.


 
 Return to top of page · Post #: 3 · Written at 10:38:06 AM on 25 September 2020.
Trobbins's avatar
 Location: Melbourne, VIC
 Member since 11 July 2012
 Member #: 1179
 Postcount: 45

An uncommon valve amp technique was proposed by Jeffery (Aug 1947 WW), where the anode load of a pentode input stage was split, and the following cathodyne stage had its cathode connected back to the split anode load via capacitor. This bootstrap arrangement effectively increases the load resistance that the pentode operates into and hence allows the input stage gain to approach very high levels. And the cathodyne stage then provides a convenient driver for a push-pull output stage.

I recently tried that with a 6EA8 triode-pentode to achieve a stage voltage gain of x370. When the pentode stage anode is directly coupled to the cathodyne grid, then this becomes a very simple circuit arrangement, although it requires a cathodyne resistor value modification to provide balanced PI operation.


 
 Return to top of page · Post #: 4 · Written at 11:01:30 AM on 25 September 2020.
NewVista's avatar
 Location: MilwWI, US
 Member since 10 May 2013
 Member #: 1340
 Postcount: 800

An uncommon valve amp technique..

I understand the McIntosh MC75 employs bootstrap also - so this technique definitely not confined to transistors!


 
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