Forums

Class B RF amplifier

Started by amal banerjee September 28, 2019
Can the class B configuration be used for RF amplifiers,
given its inherent problem of cross over distortion All 
hints, suggestions are welcome. Thanks in advance.
 
On Fri, 27 Sep 2019 22:08:21 -0700 (PDT), amal banerjee
<dakupoto@gmail.com> wrote:

>Can the class B configuration be used for RF amplifiers, >given its inherent problem of cross over distortion All >hints, suggestions are welcome. Thanks in advance. >
RF amps usually have a tuned output network, basically a bandpass filter that cleans up harmonics. They can be class A, B, C, D, E, or a lot of others. We just did a 14 MHz sine generator that starts with a square wave from some totem-pole switchmode GaN fets. Not sure what class that would be. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Fri, 27 Sep 2019 22:08:21 -0700 (PDT), amal banerjee
<dakupoto@gmail.com> wrote:

>Can the class B configuration be used for RF amplifiers, >given its inherent problem of cross over distortion All >hints, suggestions are welcome. Thanks in advance.
In a single-ended class B amplifier, the conduction angle is 180 decrees, in class C the conduction angle is often much less than 180 degrees. Both can be used to generate RF power when followed by an LC tank circuit, which ensures waveform continuation up to full 360 degrees. Class C is mainly usable for constant amplitude modes, such as FM, while class B is required for AM and SSB.so that the amplitude can be accurately controlled. Due to the tank circuit these are essentially narrow band amplifiers. Class B or Class AB can also be used in wideband applications without a tank, if a push-pull arrangement is used, extending the conduction angle to 360 degrees.
amal banerjee wrote:

--------------------
> > Can the class B configuration be used for RF amplifiers, > given its inherent problem of cross over distortion >
** Class B push-pull amplifiers are used in RF transmitters. X-over discontinuity can be reduced to any desired level - both with audio band amplifiers and also at much higher frequencies. Class B has the advantage of zero power drain when not driven plus *linear* operation - as alluded to by another poster - and require little post filtering of the output signal due to havening inherent low distortion. Ham radio guys like to use "linears" to boost the power of small transmitters (eg CB or hand held radios), which can them be of any kind. .... Phil
amal banerjee wrote...
> > Can the class B configuration be used for RF amplifiers, > given its inherent problem of cross over distortion All > hints, suggestions are welcome. Thanks in advance.
Yes. My AMP-70A 100-watt DC-10MHz amplifier operates in low-distortion class AB. Some people consider anything above 0.5MHz to be RF, that's where AM radio band starts. https://www.dropbox.com/s/elhs1whl2dfn0i7/AMP-70A-2_sch.pdf?dl=1 Discussed at length here on s.e.d, and if anyone needs more complete info, I can provide a different link. -- Thanks, - Win
On Sat, 28 Sep 2019 02:07:48 -0700, Winfield Hill wrote:

> Yes. My AMP-70A 100-watt DC-10MHz amplifier operates in low-distortion > class AB. Some people consider anything above 0.5MHz to be RF, that's > where AM radio band starts.
RF starts below 500kHz, though. -- This message may be freely reproduced without limit or charge only via the Usenet protocol. Reproduction in whole or part through other protocols, whether for profit or not, is conditional upon a charge of GBP10.00 per reproduction. Publication in this manner via non-Usenet protocols constitutes acceptance of this condition.
On Saturday, September 28, 2019 at 11:53:51 PM UTC+10, Cursitor Doom wrote:
> On Sat, 28 Sep 2019 02:07:48 -0700, Winfield Hill wrote: > > > Yes. My AMP-70A 100-watt DC-10MHz amplifier operates in low-distortion > > class AB. Some people consider anything above 0.5MHz to be RF, that's > > where AM radio band starts. > > RF starts below 500kHz, though.
Cursitor Doom probably has the BBC's radio 4 in mind, which transmitted on Long Wave at 198kHz, as well as at higher frequencies. Win may be thinking of US AM radio. In the circles I moved, RF was just relatively narrow band stuff that included tuned circuits. The circuits we used to create half-nanosecond wide pulses weren't seen as RF, though the bandwidth obviously had to stretch up to above a GHz. -- Bill Sloman, Sydney
Am 28.09.19 um 15:53 schrieb Cursitor Doom:
> On Sat, 28 Sep 2019 02:07:48 -0700, Winfield Hill wrote: > >> Yes. My AMP-70A 100-watt DC-10MHz amplifier operates in low-distortion >> class AB. Some people consider anything above 0.5MHz to be RF, that's >> where AM radio band starts. > > RF starts below 500kHz, though.
That's just nervous DC. G.
Gerhard Hoffmann wrote...
>Am 28.09.19 um 15:53 schrieb Cursitor Doom: >> On Sat, 28 Sep 2019 02:07:48 -0700, Winfield Hill wrote: >> >>> Yes. My AMP-70A 100-watt DC-10MHz amplifier operates in low-distortion >>> class AB. Some people consider anything above 0.5MHz to be RF, that's >>> where AM radio band starts. >> >> RF starts below 500kHz, though. > > That's just nervous DC.
Yes. I don't think of my 10MHz power amplifier as an RF amplifier. To me, RF amplifiers need to use RF techniques, and therefore look like RF amplifiers. My amplifier, even tho it stems from the 50MHz Tektronix PG508, looks like some audio amplifiers, but made with low-capacitance video transistors, running at much higher-than-normal currents. -- Thanks, - Win
On 28.9.19 08:08, amal banerjee wrote:
> Can the class B configuration be used for RF amplifiers, > given its inherent problem of cross over distortion All > hints, suggestions are welcome. Thanks in advance.
There are plenty of Class B RF amplifiers around, The cross-over distortion does not matter much if the amplifier is not needed to be linear. Most of the Class B linear RF amplifiers are actually deep in Class AB, to avoid the worst cross-over distortion. The amplifiers used for radio communications do not usually care about distortion of the carrier frequency wave, but distortion of the modulation envelope can spoil the results. The carrier frequency wave is usually restored by post-filtering the output, either with tuned circuits or with suitable low-pass filter. -- -TV