On Tue, 25 Sep 2007 08:09:50 -0700, Telstar Electronics
wrote:
|On Sep 25, 9:37 am, james wrote:
| Bovine excrement. Ever heard of equalizing of current through a series
| resistance? Often called a ballast resistor. Used quite often in power
| transistors where there are several base/emiter pn jumctions.
|
|OK James... less not discuss ballasting... because that has no bearing
|in this particular discussion. Maybe you can tell us how the diode
|method of biasing shown at
http://www.ifwtech.co.uk/g3sek/tr-bias/tr-bias1.htm
|(bad circuits section) works. And since you think that circuit works
|well. Please tell us why.
|
www.telstar-electronics.com
|------------
Actually quite simple if you understand nodal analysis. Per Kirchoff's
Laws the total current leaving a node must equal the current entering
a node. The particular node that is of interest is in bad circuit A,
is the junction of VRI, D1 and the base of the transisitor. For DC
analysis the cap is an open circuit and the RFC is essentially a
short. The current through D1 is set by the value of VR1, the supply
voltage and the internal pn junction of the diode. The voltage drop
across D1 is determined by the current flowing through the diode. In
turn this sets the Vbe voltage of the base/emmitter PN junction. The
instantaineous base/emmiter current varies with RF drive level. Should
this current exceed 1/10th the current through D1, then D1 starts to
loose regulation. This is the basic workings of a shunt regulator. Any
good first year tech should know this.
For either circuits, bad A and B, to work properly, the D1 diode
current should be ten times the peak instantaineous base current of
the RF power transistor. This type of biasing is good for low power RF
devices. When dealing with large power devices, ie in the 100W and
more class, this is not a good bias as the wasted heat in VR1 and D1
is very inefficient and prone to failure. Also there is no protection
should a voltage spike on the DC supply propogate through. Most
bipolar RF device do not like more than about 4 VDC base/emmiter
voltage. Both circuits have no provisions for limiting the bias
voltage in such a manner to cause permanent damage to the base/emitter
junction.
james