AN-222| Application Note
Super Matched Bipolar Transistor Pair Sets New Standards for Drift and Noise
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 mo
nolithic 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 uV over a collector current range of 1 uA 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 40 and 0.4 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-on
e 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 0.4 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:
National Semiconductor Application Note 222 July 1979
and between the two halves of the LM194 where collector currents are unbalanced. Of particular importance is the fact that the kT/q logarithmic constant is an absolute quantity dependent only on Boltzman's constant (k), absolute temperature (T), and the charge on the electron (q). Since these values are independent of processing, there is virtually no variation from unit to unit at a fixed temperature. Lab measurements indicate that the logarithmic constant measured at a 10:1 collector current ratio does not vary more than 0.5% from its theoretical value. Applications such as logarithmic converters, multipliers, thermometers, voltage references, and voltage-controlled amplifiers can take advantage of this inherent accuracy to provide adjustment-free precision circuits.
Approaching Theoretical Noise
In many low-level amplifier applications, the limiting factor on performance is noise. With bipolar transistors, the theoretical value for emitter-base voltage noise is a function only of absolute temperature and collector current.
This formula indicates that voltage noise can be reduced to low levels by simply raising collector current. In fact, that is exactly what happens until collector current reaches a level where parasitic transistor noise limits any further reduction. This "noise floor" is usually created by and modeled as an equivalent resistor (rbb') in series with the base of the transistor. Low parasitic base resistance is therefore an important factor in ultra-low-noise applications where collector current is pushed to the limits. The 40 equivalent rbb' of the LM194 is considerably lower than that of other small-signal transistors. In addition, this device has no excess noise at lower current levels and coincides almost exactly with the predicted values. A low-noise design can be done on paper with a minimum of bench testing. Another noise component in bipolar transistors is base current noise. For any finite source impedance, current noise must be considered as a quadrature addition to voltage noise.
where rs is the source impedance In the LM194, base current noise is a well-defined function of collector current and can be expressed as:
This relationship holds true both within a single transistor where IC1 and IC2 represent two different operating currents
AN-222
2002 National Semiconductor Corporation
AN006922
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