PNP Characteristic Curves

Measured Characteristic Curves for 2N3906

for I_B = -10 µA, -20 µA, -30 µA, ... , -80 µA

big data file, smaller data file, postscript plot, pdf plot

Note that you can make the PNP characteristic curves look like the common NPN curves just by rotating the PNP plot by 180°. In swapping N for P so NPN PNP, we've reversed the direction of current flows (so currents are negative -- flowing out of the collector and base in a PNP) and the required supply voltage becomes negative for a PNP.

That is to say that the PNP is designed for negative power supplies and out-flowing (negative) base and collector currents -- the opposite of NPNs.

The behavior of a PNP bipolar transistor is largely controlled by the current flowing out of the base. For the usual collector-emitter voltage drops (i.e., the active region: negative voltages from a fraction of a volt down to some breakdown voltage) the collector current (I_C) is nearly independent of the collector-emitter voltage (V_CE), and instead depends on the base current (I_B). (This is unusual behavior: usually more voltage produces to more current, but here the current only increases slightly with increasingly negative V_CE.) The current gain, i.e., the ratio of the collector current to the base current, is often denoted by or h_FE:

= h_FE = I_C/I_B

Thus in the simplest approximation the characteristic curves of a PNP are a set of flat, evenly spaced, lines:

Each (flat) curve shows that I_C doesn't change with changing V_CE. The different levels show that I_C does depend on I_B.

A slightly more complicated approximation takes into account the sloping characteristic curves through a constant Early Voltage (V_A). Here we assume that the characteristic curves all have a common x-axis intercept at the large positive voltage V_A. (The dashed curves are far from the active region and in no way represent the actual behavior of the transistor for positive V_CE. In fact, the PNP transistor is not designed to be operated with positive V_CE.)

(For the above measured 2N3906, the Early voltage ranges from 40 to 50 V. That is the extrapolated characteristic curves do not intersect at a point.)

The actual relationship between the collector current (I_C) and the controlling base current (I_B) and collector-emitter voltage drop (V_CE) is some complicated function which we can denote:

I_C(I_B,V_CE)

Like any function we can approximate it near a particular point using just the first terms of a Taylors expansion:

Clearly these hybrid (h) parameters are not constants. For example h_oe is the slope of a characteristic curve, which is nearly zero for small |I_B| and increases for more negative I_B. (Note that slope on an I-V is basically the inverse of the resistance. Thus 1/h_oe can be described as the output impedance. A typical value for 1/h_oe would be 100,000 .)

Even the defining parameter -- the current gain or h_FE -- is not exactly constant and depends on I_B. Here is the measured relationship for the above 2N3906:

The characteristic curves focus on the output of the transistor, but we can also consider the behavior of the input. In the active region the base is a forward biased diode, and so V_B would be about -.7 V, typical for a conducting Si diode. Of course in greater detail the relationship between V_B and I_B would be given by the Shockley diode equation:

where I_s is a constant and the thermal voltage V_T is given by:

Because of the exponential relationship between base current and base voltage, the slope of this relationship (which could be called the input conductance, or 1/r_B, or 1/h_ie) can be approximated by:

where in the last equation (25 /I_B) the absolute value of the base current must be entered in mA.

A desirable characteristic of a transistor is that the outputs have little effect on the inputs, but if we look in detail we find that V_CE affects V_B. The actual functional relationship giving the base voltage from the base current (I_B) and collector-emitter voltage drop (V_CE) is some complicated function which we can denote:

V_B(I_B,V_CE)

Like any function we can again approximate it near a particular point using just the first terms of Taylors expansion:

The small value of h_re shows that the input is largely unaffected by the output.

The spec sheet reports the following values for the 2N3906:

Characteristics	Symbol	Min	Max	Unit
Input Impedance	hie	2	12	k Ohms
Voltage Feedback Ratio	hre	0.1	10	×10-4
Small-Signal Current Gain	hfe	100	400
Output Admittance	hoe	3.0	60	µ mhos