have the biggest uncontrollable effect on a regulator, especially when going from a cold turn-on to the hot chassis of a tube amplifier. The temperature range under a chassis can easily be up to 50ºC. For a 300 volt supply and a temperature coefficient of 0.1% per °C (typical for a high-voltage zener or cheap 3-terminal regulator), the output drift would be 15 volts! Note that the temperature coefficient of some resistors, even precision metal film types, can be large enough to destroy the accuracy of a regulator. For a good regulator, resistors with known temperature behavior should be used.
    The main varieties of references in both the low and high voltage categories will be reviewed, then detailed application information will be given on the two most common references: zener diodes and gas tubes.

    The zener diode is the most basic semiconductor reference. It depends on the Zener effect below about 5 volts and on avalanche breakdown above about 5 volts. It is a constant voltage shunt-type device - in other words, above its breakdown voltage, it will try to maintain a constant voltage by drawing as much current as needed. A series current-limiting resistor is thus required. In the zener breakdown range, the temperature coefficient is negative and in the avalanche breakdown range is positive. Near 5 volts, the temperature coefficient is zero, but this zero drift point is not well defined and varies with current.
    To get a more accurate reference, special voltage reference diodes were developed that combined a regular diode and a zener in the same package. These are carefully matched so that their temperature coefficients cancel. These diode come in fixed voltages and have temperature coefficients from .01% per °C to .0005% per °C. Common voltages are: 6.2V (1N821-829), 8.4V (1N1530, 1N3154-3157), 9.0V (1N935-939), and 11.7V (1N941-945).
   With the advent of integrated circuits, more sophisticated references were possible. Some used temperature-compensated zener diodes with an output voltage of 5V (AD586), 6.9V (LM329, LM399), or 10V (AD-587). Others took advantage of the very good transistor matching possible in an IC to implement a "band-gap" reference.
   The basic band-gap reference voltage is 1.2V, and some ICs produce this directly (LM113, AD589). Most use internal op-amps and laser-trimmed resistors to give standard reference voltages: 2.5V (LM336, MC1403, AD580, REF-03), 5.0V (LM336, AD586, REF-02), and 10V (LM369, AD581, REF-01). These types have temperature coefficients from .01% per °C to as low as .0001% per °C. They also have lower output impedance than regular zeners and if laser-trimmed, can have an absolute voltage accuracy ranging from + per -4% to as low as .01%.

Low Voltage References
    The most basic reference is a battery. If used solely as a reference (with almost no power being drawn from the battery), its life is essentially its shelf life. In the past, mercury batteries were sometimes used as voltage references, since they had a long shelf life and very stable output voltage. These are no longer available due to their environmental hazards, but modern lithium and alkaline batteries have shelf lives of at least several years and fairly stable output voltage. Batteries should not be exposed to high temperatures, and provision should be made for checking that they are not run down. Batteries are seldom seen in tube regulator circuits, although they can be effective as a fixed grid bias source. The battery voltage depends on the cell chemistry, and ranges from 1.4 volts for an alkaline cell to 3.6 volts for a lithium cell.