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What is the circuit frequency?

I'm reading the Art of Electronics. Before I read 'TAB Electricity and ....'

I wonder how they set the circuit frequency, for example it declares the circuit frequency is 100Hz. Then, explain how to set values for the capacitors, resistors, Impedence, etc.

The books are very shy about the frequency related stuff. They just dump the frequency for the circuits then explain how to set f3dB and low-filter, high-filter, Impedence, etc.

Except setting up the frequencies, I've understood the basic analog circuits alot better than before. But, it bothers me that why should I memory the frequency without the base theories.

Am I missing a big point about setting the frequencies or it's too technical for introductory book?

Reply to
nabi
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I can only guess, without having a specific example in front of me. I guess that they are proposing a hypothetical situation. Assume that you want a circuit that has a corner frequency (between the pass band and the stop band of X hertz. Then an example is given, and you are supposed to scale the components to any other frequency you need.

Have you got a page number in the Art of Electronics for me to look at?

Reply to
John Popelish

I have 2nd edition. It's page 84, Figure 2.37.

Student Manual, page 115 Ch2: Worked Example: Common Emitter Amplifier ..following specs: - V+ = 20V - f3dB = 100 Hz - gain = -100 at quiescent point - Ic quiescent = 0.5 mA

specific example in front

Reply to
nabi

In this case, an arbitrary set of specifications is given, and the components that achieve those specifications. They are saying, "assume you are given the task of designing an amplifier to these specifications." One of those hypothetical specifications is that the gain falls 3 db as the frequency falls below 100 Hz. The gain spec requires that the total emitter resistance (both internal to the transistor and in series, externally, and combination with the DC bias emitter resistor, I'll call Regain... see circuits 2.39 and 2.40 for the configurations) must be about

1% of the collector load resistance. In order that the gain fall by 3db requires a capacitor in series with the effective emitter gain setting resistance that has a magnitude of impedance just about equal the gain setting emitter resistance, Regain at 100 Hz.

You would solve for the capacitor that has a magnitude of impedance of Zc=1/(2*pi*f*C), Regain=1/(2*pi*100*C), so C=1/(2*pi*100*Regain). Different low frequency corner specification or effective emitter resistance in series with the capacitor would produce different results.

Have I made any sense?

Reply to
John Popelish

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