The photomultiplier: DC amplification
 
In this technique, the current is made to flow through a high-valued resistor and the voltage developed across it is then amplified. A simplified scheme of the arrangement is depicted in figure 1. By using Ohm’s law, the magnitude of the voltage developed for any desired further amplification can be estimated. Thus, if the current from the anode is of the order of 10⁻¹² A and the value of the resistor is 10⁹ Ω, the voltage developed is given by
V = 10⁻¹² × 10⁹
= 10⁻³ V.

Figure 1. The DC amplification technique.
It is usual practice to keep the gain of the DC amplifier fixed and to allow for a wide range in signal levels by having a series of high-valued resistors and a selector switch. Accurate photometry can only be performed if the gain of the DC amplification is drift free. The final output signal can be displayed on a pen recorder or integrated and displayed digitally. With these types of system, the accuracy limit of the photometry is set by the electronic circuitry or the mode of display and it is usually possible to obtain a good value to 1%.