Amplifier analyzing methodFrom a given amplifier circuit first do -the dc analysis (recall lesson 6) and than do the ac analysis.
DC equivalent circuit For dc, all capacitors are acting like open switches; therefore we can draw the following dc equivalent circuit:
Now the dc analysis can easily be done: (see Lesson 6) V_{B} = 1.8V AC equivalent circuit For the ac all capacitors are shorted and the dc sources are reduced to zero:
The top of the 10K and 3.6K resistors are grounded. The resistors 10K/2.2K and 3.6K/10K are in parallel so we can combine them:
Now we got a really simple circuit for the ac analysis. Voltage Gain One of the most important characteristics for small signal amplifiers is the voltage gain (A_{V}). _{} The lowercase letters are used to indicate ac values. The output voltage is given by: V_{out} = i_{c} * r_{c} The input voltage is given by: V_{in} = i_{e} * r_{e} Substitute of these two expressions: _{} Because i_{c} approximately equals i_{e}: _{} AC emitter resistance (r_{e}) The first step in calculating the voltage gain is to estimate the ac emitter resistance (r_{e}). _{} (formula derived by using calculus) This relation applies to all transistors that means it is a universal formula. Let's remember our example circuit (Fig. 7-4): _{} AC collector resistance Due to the ac analyzing method we easily get the ac collector resistance (re). See Fig. 7-4: r_{c} = 2.65KΩ So now we are ready to calculate the voltage gain: _{} HO: What will be the voltage gain for the following circuit?
Solution: DC analysis
AC analysis r_{c} = 2.65KΩ _{} _{} |
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