Bias Circuit for Bipolar Transimpedance Amplifier

Bias Circuit for Bipolar Transimpedance Amplifier

      This article describes a bias circuit for two
configurations of transimpedance amplifiers.  This bias circuit
significantly improves the following parameters of these
transimpedance amplifiers:  linearity of the phase-frequency plot,
magnitude and bandwidth of the transimpedance, electrical noise, and
the tolerance of the transimpedance over variations of temperature,
power supply, and process parameters.

      Figure 1A shows one of the prior-art configurations of
transimpedance amplifiers.  It consists of a common-emitter stage
followed by a common-collector stage, and, hence, this is knows as
the CE-CC configuration.  Diode-connected transistors T3 and T4 in
series with feedback resistor RF raise the output DC level to about
midpoint between ground and the power supply.  By reversing the order
of these stages, the CC-CE configuration of Figure 1B is obtained.
This usually needs an additional output stage T3 to drive capacitive
loads.

      For high-beta T1, the voltage drop in RF can be neglected and
the collector current of T1 is about
      IC1 = (VS - 4*VBE)/RL                        (1)

      This is important because all of the amplifier parameters
mentioned above depend on IC1, RL, or both; namely:  phase linearity
improves with wider bandwidth which, in turn, goes with lower RL and
IC1.  Electrical noise decreases with lower IC1.  The transimpedance
depends on the voltage gain of the amplifier which, in turn, is
sensitive to variations in power supply, temperature, and process
and, consequently, the transimpedance also is. Then, for an improved
transimpedance amplifier, lower IC1, RL and a less sensitive IC1 are
required.  The first two requirements cannot be realized with
Equation (1) where a lower IC1 calls for a higher RL.

      The bias circuit to improve CE-CC and CC-CE transimpedance
amplifiers is shown in Figure 1C biasing the former.  Its main
advantages are that it makes possible reducing IC1 without increasing
RL and that it reduces the sensitivity of IC1 to temperature, power
supply, and process variations.  An equation for IC1 of the
tra...