Re: LF: Why?

[MarkusVester@aol.com]

Dear J. and LF,

... this is a very interesting question, exposing the heart of the matter. To my understanding, the presence of this little bypass capacitor decides between two fundamentally different mdes of Class-D operation.

In "voltage-mode class-D" with the center tap RF-grounded, the drain voltages are ideally squarewaves between zero and 2x Vdd, with one transistor looking like the mirror image of the other. The drain currents are half sines, just like in class-B. The output filter has to present a high input impedance for all odd harmonics, so a T-type lowpass (or a series resonant circuit) should be used. The transformer itself and the wiring must have very low stray inductance to suppress high frequency ringing, caused by a common-mode parallel resonance with the FET output capacitances. Stray inductance can be kept low by bifilar winding of the transformer primary. Alternatively, it can be completely avoided in a single-ended pushpull (halfbridge) configuration, which does not require a balun transformer and shunts all even harmonics directly - this is what I have been using in my 200 W PA for six years now.

Without the bypass at the tap, you enter the "current-mode class-D" regime, where the constant current from the choke is simply switched between the transistors. Thus each FET delivers a squarewave current contribution. The output filter has to shortcircuit the odd harmonics, as in a Pi or parallel tank configuration. The off-state drain voltages are ideally halfcycle sinusoids, peaking at pi times Vdd. The common mode voltage developed across the choke consists of even harmonics and looks like the output of a full wave rectifier. The transformer's stray inductance is not a problem here as it is in series with the choke anyway, so bifilar winding is not needed.

Apart from these basic considerations, perhaps Jim or David could comment on their views of the advantages of one mode or the other?

Kind regards,
Markus, DF6NM


In einer eMail vom 31.08.2006 19:47:07 Westeuropäische Sommerzeit schreibt vy1ja@northwestel.net: 

> Thinkers,
>
> What are the reasons for not bypassing the center tap on the push-pull amps 
> apart from high bypass current?
>
> There are spikes on the rising edge near the positive and negative peaks in 
> the waveform of the output from the LPF from the amp I am working on, and 
> bypassing the center tap to ground cleared up the spikes.  It also cleared 
> up the bypass capacitor  :o).
>
> J.