Graphical Analysis of a BJT
We can use graphical analysis to approximately analyze the response of simple transistor amplifier circuits. This technique is primarily useful to develop physical insight. Consider once again the “conceptual BJT amplifier” circuit:
Similar to the analytical solution, there are two primary steps to the graphical solution of such small-signal amplifiers:
1. DC basis analysis
2. AC small-signal analysis.
The first step in the bias calculations is to determine IB. This is done with the iB-vBE characteristic curve and the load line:
Once IB has been determined we can compute IC knowing that Ic=b IB for a BJT in the active mode. With this IC value and the iC-vCE characteristic curve of the transistor, we can determine V We haven’t yet seen the iC-vCE characteristic curve of the BJT. This can be measured using the circuit in Fig. 5.19(a) below. vBEis fixed at some value, then vCE is swept while measuring iC. The results are shown below for different values of vBE.
When vCE is very small, iC is nearly zero. This is the cutoff mode of the BJT. As vCE increases, the CBJ is forward biased and the BJT is in the saturation mode. When vCE becomes large enough, the CBJ becomes reversed biased and the BJT enters the active mode. The slopes of the lines in Fig. 5.19 in the active mode are quite exaggerated in this figure. So, back to the graphical solution. With the IC=bIB value from Fig. 5.28 and the iC-vCE characteristic curve of the transistor from Fig. 5.19, we can determine VCE:
Curve tracers are pieces of equipment that will measure and display families of iC-vCE characteristic curves for transistors.
AC Small-Signal Analysis
The first step in the AC small-signal analysis is to determine ib. This is performed using a slightly complicated interaction of the input waveform vi, the subsequent time variation of the load line, and the iB-vBE characteristic curve of the BJT:
From this comes the small-signal quantities vbe and ib. With ib known and ic=biB , then we use these values on the vic characteristic curve to determine vce:
Cutoff and Saturation
Notice that there are limits on vCE in which the BJT remains in the active mode:
• Too large ( CC V = ) and the BJT cuts off
• Too small (few tenths of a volt) and the transistor
saturates. These limits are readily apparent if we reexamine the previous figure of the small-signal variation:
Because of these limits on vCE, it is important to choose the Q point properly to all for the desired swing in the signal voltage (vce).