A problem which has plagued design engineers for years is that of designing a stable Class A solid-state high power amplifier. A Class A amplifier is one in which the quiescent current that is used to bias the output devices of the amplifier is set so that the maximum signal swing is still within the normal operating range of the transistors (i.e., within the linear range of the transistors). Due to the characteristics of the transistors themselves, it has been almost impossible to maintain a stable bias current under all operating conditions. The problem stems from the fact that there are two temperature dependent parameters which generally come into play in solid-state Class A amplifiers, as well as other types of amplifiers. Initially, the base to emitter voltage (V.sub.be) of a silicon transistor changes at a rate of 2 mv/.degree.C. Secondly, the Beta (hfe), which is the forward current transfer ratio (or gain) of a transistor, will increase with temperature up to a peak and then rapidly fall. The maximum point is known as the current density knee. The quiescent bias current I.sub.c of a transistor is a function of the two temperature dependent variables, i.e., the base to emitter voltage and the Beta of the transistor. The base to emitter voltage/I.sub.c relationship is an exponential one. Therefore, a large change in temperature will result in an exponential change in the bias current of the transistor, thus causing instability.
Another problem in the design of Class A transistor amplifiers is that the thermal time constant (i.e., the time from a step change in power dissipation until the junction temperature reaches 63.2% of the final value of junction temperature change) of a silicon transistor is such that a short-term temperature rise can occur in the chip of the transistor within a period of about 10 milliseconds. This in turn can cause a large change in the quiescent bias current which in turn can lead to thermal runaway, causing the transistors to self-destruct before any corrective circuits have time to react. Thermal stability may be achieved in a Class A amplifier by controlling the quiescent bias current. However, since the quiescent bias current is a function of two temperature dependent variables, such control is extremely difficult. If one or both of these variables could be eliminated, it would be much easier to control the quiescent bias current.
Heretofore, several methods have been used to provide bias current stability. One of these is passive thermal coupling between the transistors and the bias network. In this approach, the bias network which controls the bias current is bonded as close to the output devices on the heat sinks of an amplifier as is possible, so as to share thermal equilibrium. This approach has several problems, however. One of these is that the heat sink itself may have a variable thermal gradient, thus preventing thermal equilibrium from being achieved. Secondly, the output transistors themselves may have a variable junction to case thermal gradient which would prevent thermal equilibrium from being achieved. Finally, the tracking characteristics of the bias network may be such that they cannot track a rapid change in temperature, and thus bias current, with the result that thermal runaway occurs. These problems may be overcome by operating the output transistors beyond the current density knee in the Beta vs. temperature curve. Since the Beta of a transistor decreases with an increase in temperature beyond the current density knee, thermal runaway might be prevented in such case. Tradeoffs associated with this type of operation, however, are that the output transistors are operated at a high temperature, which may lead to short life, and that the transistors are operating near their nonlinear region, with a resultant increase in distortion. In addition, such designs usually employ large stabilization resistors resulting in a substantial waste of power.
A second method of achieving bias stability in Class A amplifiers has been to actively sense and control the bias current by servo action. In this scheme, an active negative feedback circuit is used to sense the output stage quiescent current and provide control over it. Such circuits have an inherent time delay in the corrections provided by them, however, with the resultant possibility that thermal runaway might occur before the feedback circuit has time to make corrections.
In view of the above, it is a primary object of the present invention to achieve a design for a Class A transistor power amplifier which has absolute quiescent current stability under the worst possible environmental conditions of ambient temperature rise.
It is a further object of the present invention to achieve bias current stability while at the same time having an amplifier of low distortion, wide bandwidth and high power.
It is another object of the present invention to eliminate the effect of changes of the transistor parameter of base to emitter voltage with a change in temperature in the design of solid-state Class A amplifiers.
It is a further object of the invention to achieve bias current stability by completely passive means.
It is another object of the invention to eliminate any time constants in bias current control so as to permit instantaneous control of bias current and prevent thermal instability.
It is a further object of this invention to achieve a design for a Class A amplifier which is inherently linear. Specifically, the design results in an amplifier having an open loop distortion of less than 0.1% from 20 Hz to 20 kHz when the amplifier is operated below clipping. This distortion may be further reduced by the application of negative feedback.
It is another object of the present invention to design a Class A transistor power amplifier which operates well within the linear region of the output transistors of the amplifier.
It is another object of the invention to provide an amplifier having inherent immunity from short circuit problems without the need of additional protective circuitry.