Field effect versus bipolar transistors

[Andre' Kesteloot]

James Moritz wrote:

> Dear Dick, LF group,
>
> Not too sure about linear amplifiers, but FETs are certainly better
> for the switching - type LF PAs that are currently popular. The
> reason for this is similar to those given for the use of MOSFETs in
> switching power supplies, which have much in common. These are:
> -Switching speed an order of magnitude greater, giving lower
> switching losses
> -The base current drive waveform must be carefully controlled for
> optimum performance from bipolars, while MOSFETs only require
> an "on-off" square wave from a low impedance source.
> -MOSFETs do not suffer from  "secondary breakdown"
> phenomena, which means that the transient voltages and currents
> in a bipolar PA need to be more carefully controlled to prevent
> failure due to the formation of hot spots within the transistor. This
> makes MOSFETs more robust against spikey switching waveforms.
>
> The situation seems more complicated with "linear" amplifier
> designs. Significant problems arise because a linear amplifier
> inherently dissipates power, wheras most modern power
> MOSFETs are designed to operate as high efficiency switches
> with low power dissipation. This results in devices with high current
> and voltage ratings being produced in small packages, which are
> unable to dissipate the heat produced in a linear application, and
> so must be operated at much below their ratings. This in turn
> produces problems with biasing - a much-vaunted advantage of
> MOSFETs is the negative temperature coefficient of the drain
> current vs. gate bias voltage characteristic; the current falls as the
> device gets hotter, so supposedly ensuring stable biasing, easy
> parallel operation etc, etc. In fact, this only applies when the
> device is operating relatively close to it's maximum current; when
> used at lower currents, as you are forced to do by the power
> dissipation issue, the tempco is in fact positive, and thermal
> runaway  can occur as with bipolars. The Mosfets designed for
> audio PAs are much better in this respect, but cost more, and are
> only available in relatively low ratings.
>
> Also, there seems to be a dearth of reasonably high frequency,
> high power, low cost bipolar devices, whilst MOSFETs get cheaper
> all the time, and are inherently fast enough without needing to go to
> expensive, specialised RF devices
>
> I think the main problem with MOSFET PAs at the moment is that
> not all design issues have been addressed properly yet. For
> example, the transformer push-pull linear and class D designs are
> prone to produce big voltage spikes, which mean death for any PA
> device. The Decca circuit and derrivatives demonstrate that robust
> and reliable designs can be achieved, however.
>
> Cheers, Jim Moritz
> 73 de M0BMU