# Biasing in general

1. Aug 4, 2008

### piyudhaker

I was studing about baising in general (irrespective if it is for BJt or MOS) and just looking at the graph i had several questions.

I know the basic concept, the amplifiers, +ve and -ve saturation limits causes the signal outside the limit to truncate. to ensure that the signale should not truncate, we need to determine the range of input signals. Also in order to get the linear relation between input and output we need to select the range.

We bias the circuit to ensure that the signal is within the range and we is within the linear region.

My qestion is, does biasing depend on the signal frequency? Does it changes for signals with higher frequency and signals with lower frequency?

Also, I would really appreciate if you could help me in explaining how you got the results?

which books did u refer or what logic did you follow.

Thanks,
Regards!!!

2. Aug 4, 2008

### ranger

Hi piyudhaker and welcome to PF!

High frequency signals are truly interesting wrt semiconductors. Component behaviors change (dramatically) at high frequencies, owing to the fact that they are not ideal. Seemingly insignificant junction capacitance and inductance will become an issue to deal with as the frequency increases. There is really a whole lot more to it than this. But I cannot do much justice.

One of my favorite books on high frequency stuff is "RF circuit design" by Chris Bowick. Its an old book though, around 1982. But I'm apparently seeing a version published 2008. I didnt get a chance to read the newer one and hope to get to it soon. Maybe your school library has a copy of this book, if so check it out as it will answer most of your questions about HF and semiconductors.

3. Aug 4, 2008

### rbj

even though a transistor behaves differently at extremely high frequencies (it sorta craps out) that it does at low frequencies, generally the biasing model of the transistor is not much different with one notable exception: the DC load line vs. the AC load line. the former is what you get when you assume the coupling capacitors are open circuits and the latter is what you get when you assume the coupling capacitors are ideal DC voltage sources (with no series resistance) equal to their bias voltages. the instantaneous operating point of the transistor swings away from the quiescent point (called the "Q-point", which lies at the intersection of the DC and AC load lines) along the AC load line, not along the DC load line. what you normally want to do is locate the Q-point midway between the saturation region and the cutoff region along the AC load line in order to maximize the signal "swing" before there is clipping on either the bottom or the top of the AC waveform.

4. Aug 4, 2008

### Staff: Mentor

You will often use a different kind of biasing at higher frequencies. You will switch from simple biasing to Cascode biasing:

http://en.wikipedia.org/wiki/Cascode

Quiz Question -- Why? (Let the OP try to answer first....)