Cascode

The cascode circuit ( not to be confused with the cascade circuit ) is an electrical amplifier circuit having at least two electron tubes or transistors, which are connected in series. The main advantage is the negligible Miller effect, which allows a higher bandwidth, but also a high input and output impedance.

The word is a portmanteau and cascode originated in 1939 from the term " cascade to cathode" what the series connection of the two cathodes is to indicate in the electron tubes originally used. The circuit was developed in 1939 in a paper by F.V. Hunt and R. W. Hickman first described for voltage stabilization.

Structure with tubes

In the past implementation, electron tubes, consisting of two triode, wherein the first step - a normal cathode base circuit - is electrically coupled to the second level, a lattice base circuit. Thus, this circuit combines two advantages: high input resistance at vanishingly little effect because the control grid of the second tube acts as a shield. Triode as are used, the noise is considerably lower than in pentode why this shape ( still equipped with electron tubes ) as an input stage of the first television receiver has been used.

Construction with transistors

The cascode circuit can also be formed with transistors as in the adjacent circuit diagram with bipolar transistors. Here, the input transistor T1 in the emitter circuit and the output transistor T2 in the base current control circuit operates.

The second stage, the transistor T2 is in the base circuit has, at the emitter terminal has a low input resistance, whereby a negative low voltage gain of the first stage and results in the Miller effect is greatly reduced. The absence of feedback of the output is greatly reduced to the input especially in the high frequency range and the usable bandwidth of RC amplifiers in oscilloscopes increases significantly. The maximum output resistance is increased while the input resistance at DC is almost identical to the emitter circuit.

The lower amplifier element must have a low reverse voltage, while the upper may have a high blocking voltage in a low current gain. That suits the specifications of many amplifier elements when it comes to high voltages to switch quickly. Optimum properties are achieved if a power MOSFET T1 is selected.

Combinations of a transistor (bottom) and the electron tube (above) are possible, the transistor reaches a voltage gain of about -100. Thus, it is possible to switch high voltages up to several 10 kV, without the high level of the control voltage of the tube to have (some 10 V) form.

Dual- gate MOSFETs are integrated series circuit of two field effect transistors; they have so little repercussions that this VHF amplifier can be built without neutralization. In mixers of superheterodyne receivers, the oscillator voltage is applied to the upper Gate2, whereby the MOSFET has similar properties to a hexode, but considerably less noisy.