Gilbert cell

In electronics, the Gilbert cell is a transistor circuit used as an analog multiplier and frequency mixer, first described by Barrie Gilbert (Analog Devices) in 1968.^[1]

. The advantage of this circuit is the output current is an accurate multiplication of the (differential) base currents of both inputs. As a mixer, its balanced operation cancels out many unwanted mixing products, resulting in a "cleaner" output.

Function

The Gilbert cell consists of two differential amplifier stages formed by emitter-coupled transistor pairs (Q1/Q4, Q3/Q5) whose outputs are connected (currents summed) with opposite phases. The emitter junctions of these amplifier stages are fed by the collectors of a third differential pair (Q2/Q6). The output currents of Q2/Q6 become emitter currents for the differential amplifiers, therefore the output currents of these stages are linearly dependent on these emitter currents and the respective input voltages. Combining the two difference stages' output currents yields four-quadrant operation.

A functionally equivalent circuit can be constructed using field-effect transistors (JFET, MOSFET) or vacuum tubes.

References

1. ^ ARRL - Wes Heyward (W7ZOI), Rick Campbell (KK7B), Bob Larkin (W7PUA) - Experimental Methods in RF Design, 2003, ISBN 978-0872598799

Applications

A Gilbert cell can be used as:

• A small signal precise four-quadrant multiplier (with condition that both inputs are small compared with V_T (Thermal voltage=0.025V)
• A large signal phase detector (with condition that both inputs are great compared with V_T)
• A modulator in a communications application (with condition that only one input is small compared with V_T while the other input is greater)
• A pre-processing circuit in a Flash ADC to reduce the number of comparators in this architecture. This is called a folding ADC.