EMRFD Message Archive 5919
Message Date From Subject 5919 2011-03-15 21:05:19 dave1e100 "Wrap-around" biasing of an NPN Hi
I refer to EMRFD Fig 2.16 on page 2.5. This shows how a PNP transistor can be used to provide "wrap-around" biasing for an NPN device. Eq 2.12 shows how to pick values for Ra, R1 and R2 and this I understand.
Where I am less sure is regarding the 3.3K resistor from the collector of the 2N3906 to the base of the NPN device. To decide on this value I am assuming you need the 'beta' for the NPN transistor. Then, by dividing the design Ic by 'beta' you can get a value for Ib. You then choose a sensible value for the collector voltage V1 of the 2N3906, say 2-3 volts less than its emitter voltage - staying out of saturation.
Then by ohm's law, the base-feed resistor to the NPN is:
Rb = (V1 - 0.6) / Ib
Does this make sense, or am I missing something?
5922 2011-03-16 11:58:34 Wes Re: "Wrap-around" biasing of an NPN Hi Dave and gang,
The PNP collector forms a current source to drive the NPN base. The 3.3K shown in Figure 2.16 is really nothing more than an impedance to isolate the RF at the NPN base from bias circuitry. It could be about any value without changing the operati
5927 2011-03-17 06:52:50 dave1e100 Re: "Wrap-around" biasing of an NPN Hi Wes
Many thanks for your help with this.
One thing I also notice with Fig 2.16 is that the NPN emitter is grounded. This would seem to imply that the use of emitter degeneration to control stage gain and impedances is no longer available.
I guess it might be ok to insert a small emitter resistor (typically, say 5-10 ohms) to reinstate this design 'handle'. Of course, with for example a 2N5109 running an Ic of 50mA and an Re of 5 ohms, you would now have an emitter voltage of 0.25V. The collector voltage on the PNP 'wrap-around' device would presumably increase by 0.25V to re-establish the equilibrium DC operating point.
However the 'gain' of the DC operating point control loop would now have increased somewhat. Both the PNP device and the new emitter resistor are now working to counteract fluctuations in the DC operating point. For the new Re: an upward perturbation in Ic will generate a greater emitter voltage across Re, momentarily reducing Vbe and hence Ib. In-turn, this will counteract the initial Ic increase. Probably, this is all ok?
So, returning to EMRFD Fig 2.16, the 0.1uF on the 2N3906 collector presumably has 2 functions:
1 To provide an RF ground for the 3.3K resistor, keeping RF out of the PNP DC control loop, as noted in your email.
2 To provide a dominant pole in the DC operating-point control loop, maintaining its stability, as noted in the caption to Fig 2.16.
Apologies to those for whom this was already 'clear as daylight' :-)