#### The Electrician

- Joined Oct 9, 2007

- 2,801

I wouldn't really expect the results from including all the h parameters to give a result any more useful in the real world than the simple approximations.You are correct of course, and a good designer needs all of this in the general case. Still, the question becomes how precise does the model have to be to understand this circuit and predict it's performance.

His analysis includes alpha=hfb and hib. It is clear that hib is critical and is highly dependent on temperature. Including alpha represents a 0.1%correction to the answer. Now including hob would be less than a 0.01% correction.

It is an exercise in circuit analysis.

If you take alpha to be 1, and use .025 mV to calculate re, then the gain at resonance is 8.000. If you include an alpha of .99 as hgmjr did, you get a gain of 7.92. This is a correction of 1%, not .1%, isn't it?

The value of 1/hob is about 13 megohms which is probably the source of your comment that including it would be less than an .01% correction. Actually, the output admittance of a transistor in CB configuration depends strongly on the impedance at the base. The parameter hob is the admittance when the base is open circuited, or driven from a very high impedance. In the case of the amplifier in this thread, at resonance the source impedance is zero, and the output resistance of the transistor is not 13 megohms, but rather about 300,000 ohms, which means including it will give about a .3% correction.

EDIT: The previous paragraph says "base" where I should have said "emitter". Thanks to Ron H for pointing this out. Here's the corrected paragraph.

-----------------------------

The value of 1/hob is about 13 megohms which is probably the source of your comment that including it would be less than a .01% correction. Actually, the output admittance of a transistor in CB configuration depends strongly on the impedance at the emitter. The parameter hob is the admittance when the emitter is open circuited, or driven from a very high impedance. In the case of the amplifier in this thread, at resonance the source impedance is zero, and the output resistance of the transistor is not 13 megohms, but rather about 300,000 ohms, which means including it will give about a .3% correction.

------------------------------

Including the reverse voltage transfer ratio, hrb, gives about a .1% correction.

Last edited: