may swing too low, causing it to saturate). Ensurance of this condition for FETs requires careful selection for the pair or special biasing of the upper FET gate, increasing cost. The cascode circuit can also be built using bipolar transistors, or MOSFETs, or even one FET (or MOSFET) and one BJT. This circuit arrangement was very common in VHF television tuners when they employed vacuum tubes. Advantages The cascode arrangement offers high gain, high bandwidth, high slew rate, high stability, and high input impedance. As a two-transistor circuit, the parts count is very low. Disadvantages The cascode circuit requires two transistors and requires a relatively high supply voltage. For the two-FET cascode, both transistors must be biased with ample VDS in operation, imposing a lower limit on the supply voltage. Dual-gate version A dual-gate MOSFET often functions as a "one-transistor" cascode. Common in the front ends of sensitive VHF receivers, a dual-gate MOSFET is operated as a common-source amplifier with the primary gate (usually designated "gate 1" by MOSFET manufacturers) connected to the input and the second gate grounded (bypassed). Internally, there is one channel covered by the two adjacent gates; therefore, the resulting circuit is electrically a cascode composed of two FETs, the common lower-drain-to-upper-source connection merely being that portion of the single channel that lies physically adjacent to the border between the two gates. Mixer in superheterodyne receivers A cascode circuit is very useful as a multiplying mixer circuit in superheterodyne receivers. At the lower gate the RF signal is fed to the mixer, and at the upper gate the local oscillator signal is fed to the mixer. Both signals are multiplied by the mixer, and the difference frequency, the intermediate frequency, is taken from the upper drain of the cascode mixer. This was further developed by cascoding whole differential-amplifier stages to form the balanced mixer, and then the Gilbert cell double-balanced mixer. Other applications With the rise of integrated circuits, transistors have become cheap in terms of silicon die area. In MOSFET technology especially, cascoding can be used in current mirrors to increase the output impedance of the output current source. A modified version of the cascode can also be used as a modulator, particularly for amplitude modulation. The upper device supplies the audio signal, and the lower is the RF amplifier device. A cascode may also be combined with a voltage ladder to form a high-voltage