10-21] SOLID STATE AMPLIFIERS 577 



With the circuit in Fig. 10-47, the proper phase must be maintained 

 between the signal which is being amplified and the pumping voltage. In 

 order to eliminate this requirement, another resonant circuit is paralleled 

 across the nonlinear capacitor as shown in Fig. 10-47. This circuit is called 

 the idler and its frequency is adjusted so that/i +/s = /p. The sum of the 

 signal and idle voltages will have zeros at the frequency /i +/s since 



cos COs/ + cos Wit = 2 cos \ {cOs + COi)/ cos I (cOs — COi) ^ (10-29) 



With the proper phase, then, the pumping voltage should introduce energy 

 into the system and increase the signal and idle voltages. It turns out that 

 the phase of the idle voltage automatically adjusts itself to a value allowing 

 pumping action of this kind. The effective result insofar as the signal circuit 

 is concerned is that the idling circuit appears as a negative resistance whose 

 value depends upon the value of the pumping voltage and the charac- 

 teristics of the nonlinear capacitance. By adjusting the effective negative 

 resistance to be only slightly less than the physical resistance in the signal 

 circuit, considerable very low noise amplification can be obtained. These 

 results are possible at room temperatures; it is not necessary, as it is in the 

 case of MASER amplifiers, to cool such a device to get very low noise 

 figures. 



In addition to the negative resistance type of parametric amplifier 

 discussed above, in which the input and output frequencies are equal, it is 

 possi,ble to couple into the idling circuit to provide an amplifying up- 

 converter. If the output resonant frequency is greater than the pumping 

 frequency, the device will be unconditionally stable (unlike the negative- 

 resistance type of device, which could become oscillatory) although the 

 maximum possible gain is limited. 



Currently, the most successful and promising types of solid-state 

 parametric amplifiers have been constructed using semiconductor junction 

 diodes as nonlinear capacitive elements. The depletion layer formed at the 

 p-n junction acts like a capacitance. Because the width of this layer is a 

 nonlinear function of the applied voltage, the capacity varies with this 

 voltage in a nonlinear fashion. 



Another method of obtaining parametric amplification at microwave 

 frequencies employs ferromagnetic materials to obtain the equivalent of 

 nonlinear inductive elements. Proposals have also been made for obtaining 

 nonlinear circuit elements at microwave frequencies by means of a number 

 of other methods. These include the use of ferroelectric effects, the Hall 

 effect, and cyclotron resonance in semiconductors. 



Fig. 10-48 briefly summarizes the present status of solid-state amplifier 

 development. The diagrams in that figure are intended to convey a 

 rudimentary physical concept of typical configurations. 



