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Class E amplifier question

Started by amal banerjee November 8, 2019
Here is a question for all electronics gurus here.
Can a class E amplifier be termed efficient - given 
the shunt capacitor from BJT collector to ground, 
which is an AC short -- unless the capacior value 
is very small, such that the capacitive reactance 
is large.  Any flaws in this argument ?

On 8.11.19 08:13, amal banerjee wrote:
> Here is a question for all electronics gurus here. > Can a class E amplifier be termed efficient - given > the shunt capacitor from BJT collector to ground, > which is an AC short -- unless the capacior value > is very small, such that the capacitive reactance > is large. Any flaws in this argument ? >
The capacitor is a part of a tuned circuit, so the simple short circuit idea is moot. I have a kilowatt RF amplifier, where there is a capacitor from the tube plates to ground and another across the output (Google for a pi-filter). -- -TV
On Thu, 7 Nov 2019 22:13:06 -0800 (PST), amal banerjee
<dakupoto@gmail.com> wrote:

>Here is a question for all electronics gurus here. >Can a class E amplifier be termed efficient - given >the shunt capacitor from BJT collector to ground, >which is an AC short -- unless the capacior value >is very small, such that the capacitive reactance >is large. Any flaws in this argument ?
A transistor dissipates instantaneous power Vce*Ic, so discharging a charged capacitor heats the device. The trick to get efficiency is to only turn on the transistor when Vce is low, namely pump a lot of current into a resonant load for part of the cycle, as the resonator voltage swings close to ground. The resonant load gets pumped by the full supply voltage times the current, a lot of power, but the transistor dissipation is low, high Ic but low Vce while it's on. Resonant power supplies aren't called class E, but work the same way. Some are called "zero voltage switching" circuits. http://www.classeradio.com/theory.htm -- John Larkin Highland Technology, Inc lunatic fringe electronics
On 11/8/2019 12:13 AM, amal banerjee wrote:
> Here is a question for all electronics gurus here. > Can a class E amplifier be termed efficient - given > the shunt capacitor from BJT collector to ground, > which is an AC short -- unless the capacior value > is very small, such that the capacitive reactance > is large. Any flaws in this argument ? >
Class E amps dramatically lower switching losses in your FET/Transistor where most of you losses are. Although, I've seen some hot inductors and transformers! This article has some numbers I haven't seen spelled out regarding efficiency and conduction angle and duty cycle. Be aware it starts as wireless energy transfer, but quickly gets to the class E numbers.
> https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=11&cad=rja&uact=8&ved=2ahUKEwjjsdqv_NrlAhVJQ6wKHTOIDOgQFjAKegQIAhAC&url=https%3A%2F%2Fwww.mdpi.com%2F1996-1073%2F10%2F9%2F1409%2Fpdf&usg=AOvVaw0bVgtR6ccI9arMQykU4JAN
Or use this.
> https://tinyurl.com/y66rlxcd
Mikek
On Friday, November 8, 2019 at 7:36:18 AM UTC-5, Tauno Voipio wrote:
> On 8.11.19 08:13, amal banerjee wrote: > > Here is a question for all electronics gurus here. > > Can a class E amplifier be termed efficient - given > > the shunt capacitor from BJT collector to ground, > > which is an AC short -- unless the capacior value > > is very small, such that the capacitive reactance > > is large. Any flaws in this argument ? > > > > The capacitor is a part of a tuned circuit, so > the simple short circuit idea is moot. > > I have a kilowatt RF amplifier, where there is > a capacitor from the tube plates to ground and > another across the output (Google for a pi-filter). > > -- > > -TV
The PI filter is not essential, i.e., the capacitor to ground(in parallel with the load) is of the low pass L impedance matching sub-circuit, and and can be replaced with a high pass(inductor to ground) L impedance matching sub-circuit, with marginal change in performance characteristics. Ub fact, the L impedance matching sub-circuit can be done away with. I have dirtied my hands a fair bit with SPICE simulations(HSpice, Ngspice) to say this.
On Friday, November 8, 2019 at 10:50:33 AM UTC-5, jla...@highlandsniptechnology.com wrote:
> On Thu, 7 Nov 2019 22:13:06 -0800 (PST), amal banerjee > <dakupoto@gmail.com> wrote: > > >Here is a question for all electronics gurus here. > >Can a class E amplifier be termed efficient - given > >the shunt capacitor from BJT collector to ground, > >which is an AC short -- unless the capacior value > >is very small, such that the capacitive reactance > >is large. Any flaws in this argument ? > > A transistor dissipates instantaneous power Vce*Ic, so discharging a > charged capacitor heats the device. The trick to get efficiency is to > only turn on the transistor when Vce is low, namely pump a lot of > current into a resonant load for part of the cycle, as the resonator > voltage swings close to ground. The resonant load gets pumped by the > full supply voltage times the current, a lot of power, but the > transistor dissipation is low, high Ic but low Vce while it's on. > > Resonant power supplies aren't called class E, but work the same way. > Some are called "zero voltage switching" circuits. > > http://www.classeradio.com/theory.htm > > > > -- > > John Larkin Highland Technology, Inc > > lunatic fringe electronics
Thanks for the explanation. Most treatments of the class E amplifier(including SPICE simulations) use an ideal switch, instead of a real BJT or FET.