r ref is dependent on desired input current i c at pin 17 of dac 102 current i c flows in the direction of the arrow at pin 17 of dac 102 shown in fig1 and thus pin 17 may be considered a current sink as is well known in the art op amp 103 is provided with a bias voltage on pins 4 and 7 and has an output at pin 6 part of which flows through r 4 to pin 17 of dac 102 and part of which flows through r 5 to pin diode 101 pins 2 and 3 of op amp 103 are inputs pin 2 being an inverting or negative input and pin 3 being a noninverting or positive input r 4 is equal to r 5 which equals 1 kohm 1 tolerance pin diode 101 is controlled provided a bias current i c 39 through resistor r 5 by pin 6 of op amp 103 and provides a linear attenuation of radio frequency rf signal 1011 fed through diode 101 additionally driving a pin diode from a current source in this manner provides excellent temperature stability again referring to fig1 companding digital to analog converter dac 102 enables the necessary exponential current i c to flow through pin 17 to provide proper current interfacing operational amplifier op amp 103 couples converter 102 to diode 101 dac 102 pins 3 to 9 accept digital control signals for simplicity only five of the seven bits are used bits 3 4 5 6 7 taking point to point data for 2 7 bits requires too much time in this circuit design pins 8 and 9 are held at 34 one 34 pin 11 serves as dac 102 linear or reference current input and determines the.