A PHOTOTUBE CIRCUIT WITH TRANSISTORS I. P. PHILLIPS and R. W. KEOWN University of Louisville, Louisville, Kentucky
IT IS commonly known that vacuum phototubes yield such small photocurrents that amplification is desirable to avoid circuits containing fragile and expensive galvanometers. Vacuum phototubes are thus more complicated for use in colorimeters than photovoltaic selenium cells, which generate large enough currents not to require amplification under ordinary circumstances. However, photovoltaic cells require low-resistance meters in order to give currents linear with light intensity and they do not have the variety of response t o portions of the spectrum that can be obtained from the different vacuum phototube types. Even though vacuum phototubes require an external power supply, their greater versatility would warrant more extensive use in colorimeters if the circuit complications of a vacuum tube amplifier could be avoided. In a recent article' a current-amplifying transistor circuit so compact that i t could be mounted inside a microammeter case was described. With matched junction transistors of the Raytheon CK-721 type and a high impedance input, this circuit, in effect, converted a 25-microampere meter into a l-microampere meter. We employed this circuit (see the figure) with an RCA 929 phototube and "B" battery to supply the input current. We expected, and did obtain, meter readings that were larger than the corresponding currents (for equal light intensity) from an unamplified photovoltaic cell. Instead of the expensive CK-721 transistors we used the cheaper but less powerful CK-722. It was necessary to do some trial-and-error experimenting to find a pair that would balance adequately. A balanced pair would give zero dark current when the ' STARKE,H. F.,Radio and Television News, 50, 82 (1953).
variable resistance, R, in the figure, was equal to the fixed resistance, R1. The hest pair we could find gave current amplification of nearly eight times; e. g., a 20microampere current read 150 microamperes on our meter (a Heathkit multimeter). With better transistors considerably greater amplification should be possible, according to the literature.'
Phototube circuit with T..Mi.tors A. 150-pa. meter.
RI. 5OWohm variable resistor. RI. 4300-ohm resistor. v. T. CK-722iunotion transis-
Rz,l-megohm resistor. BI. 45 v. Bz, 1.34 tors. P. RCA 929 phototube.
Since the cost of this amplifier, exclusive of meter, is about $10 and the construction in a breadboard fashion is very easy, the unit might be useful in a colorimeter. Such a colorirneter would be superior in sensitivity to the barrier layer cell circuits commonly available, and also would not require a meter of critically important low resistance.
