Application Note 222 Super Matched Bipolar Transistor Pair Sets New Standards for Drift and Noise

Matched bipolar transistor pairs are a very powerful design tool, yet have received less and less attention over the last few years. This is primarily due to the proliferation of high-performance monolithic circuits which are replacing many designs previously implemented with discrete components. State-of-the-art circuitry, however, is still the realm of the discrete component, especially because of recent improvements in the components themselves. It has become clear in the past few years that ultimate performance in monolithic transistor pairs was being limited by statistical fluctuations in the material itself and in the processing environment. This led to a matched transistor pair fabricated from many different individual transistors physically located in a manner which tended to average out any residual process or material gradients. At the same time, the large number of parallel devices would reduce random fluctuations by the square root of the number of devices. The LM194 is the end result. It is a monolithic bipolar matched transistor pair which offers an order-of-magnitude improvement in matching properties and parasitic base and emitter resistance over conventional transistor pairs. This was accomplished without compromising breakdown voltage or current gain. The LM194 is specified at 40V minimum collector-to-emitter breakdown voltage and has a minimum hFE of 500 at 1 mA collector current. Maximum offset voltage is 50 μV over a collector current range of 1 μA to 1 mA. Maximum hFE mismatch is 2%. Common mode rejection of offset voltage (dVOS/dVCB) is 124 dB minimum. An added benefit of paralleling many transistors is the resultant drop in overall rbb and ree, which are 40Ω and 0.4Ω respectively. This makes the logarithmic conformity of emitter-base voltage to collector current excellent even at higher current levels where other devices become non-theoretical. In addition, broadband noise is extremely low, especially at higher operating currents. The key to the success of the LM194 is the nearly one-to-one correlation between measured parameters and those predicted by a theoretical bipolar transistor model. The relationship between emitter-base voltage and collector current, for instance, is perfectly logarithmic over an extremely wide range of collector currents, deviating in the pA range because of leakage currents and above several milliamperes due to the finite 0.4Ω emitter resistance. This gives the LM194 a distinct advantage in non-linear designs where true logarithmic behavior is essential to circuit accuracy. Of equal importance is the absolute nature of the logarithmic constant, both between the two halves of the device and from unit to unit. The relationship can be expressed as: