Driver Bootstrapping Explained with Graphical Help
Altho this topology does appear to be positive feedback & increases stage gain slightly it does not qualify as feedback in the usual sense. Feedback over a single stage needs to go back to a cathode or grid where some gain can be realized. Back to the plate there is no gain possible.
The driver tube used in this example is a 6SN7GT, a very common hookup. The load resister is 27K powered by 400 Volts B+. So the center red line is a 27K loadline terminated on the 400 volt point on the 6SN7GT plate family of curves.
The bootstrap connexion inserts some signal into the power supply B+ in the same phase as the main signal. That results in a higher impedance loadline & more gain occurs. But the increase in gain is small, the driver is usually a triode.
The other red lines are the same slope as the 27K loadline but they terminate at the limits set by the boot strap signal. A new loadline shewn in green is the result. This loadline looks like ~42K, (500 / 11.8)K. Calculating gain for the 27K loadline & assuming rp to be 7.7K & mu of 20 the gain is 15.56. With the 42K loadline the gain becomes 16.90.
The difference is 20 log (16.90/15.56) db or 0.72 db. Not much at all! I measured the difference of gain with & without bootstrapping on my version of Norman Crowhurst's Twin Coupled Amplifier. It was ~1,5 db.
The real benefit of bootstrapping is the longer loadline available to drive the following stage. Taking the extremes of the possible signal excursions from the control grid at zero volts at one end & -20 volts at the other looks like the 27K loadline would be OK at 280 volts peak to peak. For the bootstrapped 42K loadine the same measurement looks like 318 volts peak to peak.
In a practical circuit the boot strap signal would be much larger than in this example. But the 6SN7 curves don't go to a high enough voltage to shew that here.
If it is connected the other way we have Schade FB. The steeper loadline increases the driver distortion.
I've added the name & address of the Bankok company that tried to build some of the bootstrapped triode amps. Probably gone now, maybe operating under a different name.
If anyone wants a PDF copy of the original 1999 article you can email me- johnnhelen4 at gmail dot com
Altho this topology does appear to be positive feedback & increases stage gain slightly it does not qualify as feedback in the usual sense. Feedback over a single stage needs to go back to a cathode or grid where some gain can be realized. Back to the plate there is no gain possible.
The driver tube used in this example is a 6SN7GT, a very common hookup. The load resister is 27K powered by 400 Volts B+. So the center red line is a 27K loadline terminated on the 400 volt point on the 6SN7GT plate family of curves.
The bootstrap connexion inserts some signal into the power supply B+ in the same phase as the main signal. That results in a higher impedance loadline & more gain occurs. But the increase in gain is small, the driver is usually a triode.
The other red lines are the same slope as the 27K loadline but they terminate at the limits set by the boot strap signal. A new loadline shewn in green is the result. This loadline looks like ~42K, (500 / 11.8)K. Calculating gain for the 27K loadline & assuming rp to be 7.7K & mu of 20 the gain is 15.56. With the 42K loadline the gain becomes 16.90.
The difference is 20 log (16.90/15.56) db or 0.72 db. Not much at all! I measured the difference of gain with & without bootstrapping on my version of Norman Crowhurst's Twin Coupled Amplifier. It was ~1,5 db.
The real benefit of bootstrapping is the longer loadline available to drive the following stage. Taking the extremes of the possible signal excursions from the control grid at zero volts at one end & -20 volts at the other looks like the 27K loadline would be OK at 280 volts peak to peak. For the bootstrapped 42K loadine the same measurement looks like 318 volts peak to peak.
In a practical circuit the boot strap signal would be much larger than in this example. But the 6SN7 curves don't go to a high enough voltage to shew that here.
If it is connected the other way we have Schade FB. The steeper loadline increases the driver distortion.
I've added the name & address of the Bankok company that tried to build some of the bootstrapped triode amps. Probably gone now, maybe operating under a different name.
If anyone wants a PDF copy of the original 1999 article you can email me- johnnhelen4 at gmail dot com
