A traditional distributed amplifier is a topology well known in industry as a proven way to build a wideband amplifier. Typical bandwidths of distributed amplifiers on a GaAs substrate could be on the order of the kHz to millimeter wave frequencies. A cascode distributed amplifier is widely recognized as a way to improve gain and bandwidth over a non-cascode distributed amplifier.
The benefit of a distributed amplifier is accomplished by incorporating the parasitic effects of the transistor into the matching networks between devices. The input and output capacitances of the device can be combined with the gate and drain line inductance, respectively, to make the transmission lines virtually transparent, excluding transmission line loss. By doing this, the gain of the amplifier may only be limited by the transconductance of the device and not the parasitics associated with the device. This only happens if the signal traveling down the gate line is in phase with the signal traveling down the drain line, so that each transistor's output voltage adds in phase with the previous transistors output. The signal traveling to the output will constructively interfere so that the signal grows along the drain line. Any reverse waves will destructively interfere since these signals will not be in phase. The gate line termination is included to absorb any signals that are not coupled to the gates of the transistors. The drain line termination is included to absorb any reverse traveling waves that could destructively interfere with the output signal.
Stability of amplifiers is essential to keep a predetermined state of the circuit. A wide variety of problems arise if the amplifier is shown to oscillate or have the potential to oscillate. Problems due to oscillation can range from fluctuations in bias conditions to circuit self-destruction, etc.
Parametric oscillation is a type of oscillation that typically only occurs when certain RF power levels are applied to an amplifier, i.e., under quiescent or small conditions the amplifier appears stable.
Prior stabilizing methods have not been beneficial in preventing parametric oscillations. For example, in a cascode distributed amplifier, there is a capacitor on the gate of the common-gate (CG) device that acts to negate the Miller Capacitance which a purely common-source (CS) amplifier contains. It negates the Miller capacitance by theoretically creating an RF short at the gate node of the CG device. In many cases, this capacitor is reduced to smaller values (˜0.5 pF) to tune the cascode circuit for improved power performance. This capacitor may require a resistor to De-Q the network such that it won't oscillate. Increasing this resistor value from a nominal value (e.g. 5 ohms) is a common way to enhance the stability of the circuit. This method, however, may hurt performance (i.e., provide much less bandwidth), and it may not help with parametric oscillation stability.
Another prior approach in attempting to stabilize the CG device of a cascode distributed amplifier is to introduce the loss seen by the drain in the form of a shunt resistor-capacitor (R-C) connected between the drains of the CG devices. This method, however, provides no significant reduction of parametric oscillation. For example, the CG gate resistance may actually become more negative while further degrading the gain of the amplifier.