The advantages of tetrode geometry for power transistors

It can be seen that the leakage currents in a common emitter triode are determined by the input circuitry and quiescent bias levels. In all cases, the lower leakage current limit is approximated by I_CBO. With triode common emitter circuitry, it is difficult to attain low values of circuit leakage without severely lowering the normal stage gain. The power tetrode differs from the triode in that it has two isolated base connections, making it possible to insert the signal through one base while using the remaining base to establish the desired no-signal leakage condition. Thus, if both bases of the tetrode are tied together, the unit behaves like a triode. If the base is connected to the emitter, the zero signal leakage current equals I_CES irrespective of the impedance level of the other base, and the transfer function of the signal base is made more linear at a somewhat reduced level and exhibits a slight input threshold. If one base is restrained by a voltage equal to the floating potential, the common emitter leakage current is reduced to I_CBO, and the transfer function remains essentially the same. Further restraint on the base will cause the input threshold to increase, a feature desirable in certain switching and control applications. Even greater advantage can be achieved with a tetrode using a temperature-sensitive bias source because, with a tetrode, the biasing circuit does not reduce the gain by shunting the input circuit and, because of the isolation, the temperature-sensing unit will not be affected by the input signal level. A practical direct-cuoupled audio amplifier is presented to illustrate the mechanisms that may be used to achieve superior performance, stability, and linearity with a minimum of expensive components. A class-A single-ended circuit is used to illustrate the linearity of the device under large signal conditions. The common-emitter cascade is chosen because of its inherent high gain and its amenability- to direct coupling.