A known passive voltage variable attenuator design, referred to as a "T Structure" attenuator, comprises two series Field Effect Transistors ("FETs") and a shunt FET connected between a point common to the series FETs and reference potential as shown in FIG. 1 of the drawings. An RFin port is an input for RF energy to be attenuated. The attenuated output appears at an RFout port. The RFin port and RFout port are opposite ends of the series FET circuit. The series FETs and the shunt FET are controlled by an attenuation control voltage source. The point common to the series FETs is held substantially at the FETs pinch off voltage. The attenuation control voltage varies the gate voltage of the series FETs. The attenuation control voltage varying the gate voltage of the series FETs controls the gate to source voltage differential causing them to operate as variable resistors over a desired attenuation range. The attenuation control voltage concurrently varies the drain voltage and source voltage of the shunt FET, while the gate of the shunt FET is connected to reference potential. Therefore, the varying attenuation control voltage also causes the shunt FET to operate as a variable resistor over the same range, but in opposite relationship with respect to the attenuation control voltage value as the series FETs. The circuit is particularly well adapted for use in a radio frequency voltage variable attenuator using GaAs MESFETs.
In the T structure voltage variable attenuator circuit, attenuation is lowest at a five volt control voltage value and highest at a zero volt control voltage value. A control voltage input biases a resistive divider network. A control voltage output of the resistive divider network is the attenuation control voltage and varies from zero to approximately pinch off voltage of the FETs as the control voltage input ramps from zero to a maximum voltage, typically five volts. A point common to the series FETs is held at the pinch off voltage by a voltage regulator circuit biased by a fixed voltage source typically five volts.
In operation a zero volt control voltage input is applied to the resistive divider network resulting in a zero volt attenuation control voltage value. The attenuation control voltage is applied to the gates of the series FETs resulting in a gate to source voltage to both series FETs at approximately a pinch off voltage value. With the gate to source voltage of the series FETs held at approximately pinch off voltage, the series FETs are essentially turned off and are in a high resistance state. The zero volt control voltage input and, therefore, the attenuation control voltage output of the resistive divider network is concurrently applied to the drain and source of the shunt FET while the gate of the shunt FET is held at reference potential. This results in a zero or very small gate to source voltage differential on the shunt FET, thereby turning it on and producing an effective low resistance in the shunt FET. The low resistance in the shunt FET creates a low resistance path to ground. RF energy applied to the RFin port is effectively shorted to ground through the low resistance shunt FET permitting very little RF energy to appear at the RFout port of the voltage variable attenuator.
At a maximum control voltage input level, typically five volts, the voltage variable attenuator is in its lowest attenuation state. A five volt control voltage input ideally produces an attenuation control voltage at or near pinch off which is applied to the gate of the series FETs. The drain and source of the series FETs held at pinch off voltage causes the gate to source voltage differential of the series FETs to be approximately zero. A zero voltage differential between the gate and source of the series FETs turns them on and places them in an effective low resistance state. Concurrently, the five volt control voltage input results in an attenuation control voltage near pinch off to the drain and source of the shunt FET. The gate of the shunt FET is held at reference potential causing the gate to source voltage differential of the shunt FET to be approximately the pinch off voltage value. The pinch off voltage differential between the gate and source of the shunt FET, turns it off and places it in an effective high resistance state. In this state, most of the RF energy applied to the RF input port of the attenuator flows through the two series FETS and appears at the RF output port because the shunt FET is an effective open circuit.
As can be appreciated by one of ordinary skill in the art, proper operation of the voltage variable attenuator requires that the attenuation control voltage ramp from zero volts to approximately pinch off voltage. It is common, however, that FETs made by the same process exhibit some variations in performance characteristics and in particular pinch off voltage. In circuits that are sensitive to pinch off voltage variations for example the attenuator, shown in FIG. 1 of the drawings variations in pinch off voltage vary the transfer function of the circuit and can adversely affect yield and repeatability. In order to compensate for the variations in pinch off voltage and improve yield in the voltage variable attenuator, a resistive divider network having three bond pads therein is included in the circuit as illustrated in FIG. 1. The resistive divider network scales the zero to five volt control voltage input of the attenuator. After determining the pinch off voltage of the FETs in the circuit, electrical connection is made to one of the three bond pads in the resistive divider network. The appropriate bond pad is the one that scales the attenuation voltage control range from the zero to five volt control voltage input range most closely to the zero to the pinch off voltage range that is used internal to the attenuator circuit. Use of the resistive divider network requires measurement of the pinch off voltage, a decision regarding the most desirable bond pad to use in the resistive divider network, and wire bonding to the appropriate bond pad in order to achieve the zero to pinch off voltage scaled control voltage output for attenuation control. This operation is time consuming, costly, and prone to error. It is also coarse and inexact in that only three variations are realizable. There is a need therefore, for an apparatus that can compensate for pinch off voltage variations and is less time consuming and more reliable than prior art solutions. The ability of the manufacturer to produce parts within a narrow performance range over time and temperature is also highly desirable and effects repeatability of the part. It is known that pinch off voltage can vary over time and temperature in some devices. For circuits that are sensitive to pinch off voltage variations, there is also a need for a circuit that will adjust and compensate for variations in pinch off voltage over time and temperature.