Current Sources

Author: Leonard Krugman

Notice that all the bias requirements are supplied by conventional batteries, which act as constant voltage sources. At this point the conscientious reader may wonder if this does not conflict with the statements in earlier chapters that transistors are current-operated devices.

Actually, the term "current source" is more than just a mathematical concept. The practical aspect can be shown as follows: Assume that a six-volt battery with negligible internal resistance is connected to a variable load resistance. Except for very low values of load, the battery terminal voltage remains constant as long as the battery remains fully charged. Now assume that a one megohm resistor is connected in series with the battery and the load resistor. In this case the current remains reasonably constant while the load resistance is varied from zero to about 0.1 megohm. Thus, the addition of a series resistor has converted the constant voltage supply into a constant current source over a fairly wide range of load resistance values. The range depends upon the value of the series resistor. Figure 5-7 illustrates the basic equivalent interchanges of supply sources. Mathematically, all that is involved is the movement of the impedance proportionality constant from one side of the equation to the other.

Fig. 5-7. Equivalent voltage-current sources.

That these circuits are equivalent can be shown by a simple example. Take the case of a six-volt battery in series with a resistor R = 1 megohm and a load R_L = 1 megohm. Then the load current equals

= 3 microamperes. The equivalent circuit on a current basis is a current generator = 6 microamperes, which is shunted by both a resistor R = 1 megohm, and a load R_L = 1 megohm. Now the load current equals the source current less the amount shunted by resistor R. Since R and R_L are in parallel, the voltage drop across each resistor must be the same, and the load current equals = 3 microamperes.

This checkswith the previous result. The same procedure can be used to convert an a-c voltage source into an a-c current source.