Discrete Bipolar Junction Transistors (BJTs) are used in countless electronic circuit designs and for many of these, the exact values of device parameters aren’t critical once they lie within a specific range. However, many applications require the use of BJTs whose parameters closely match each other. While some manufacturers offer individual BJTs whose parameters have tight tolerances, these may still not provide the level of device matching required, especially as the operating temperature of an application varies. This blog reviews some basic circuits for which closely matched BJTs are critical and explains which Nexperia‘s matched pair BJTs are ideal for these applications.
The current mirror
The current mirror circuit shown in Figure 1 is a common application for BJTs. The simplest version consists of only two devices in which the current flowing in Q2 exactly ‘mirrors’ (hence the name) the current flowing in Q1.
Having the ability to control the amount of current flowing in Q2 allows it to be used as a current source to ‘bias’ (set the operating point) other application circuits like amplifiers or comparators. In a current mirror, the base terminals of Q1 and Q2 are connected together. Assuming that both transistors are identical i.e. they have the same base-emitter voltage (VBE) and current gain (hfe) and also assuming that R1 = R2, then IOUT will exactly replicate IIN. This is be expressed mathematically as:
k1 is called the current factor and ideally, it should have a value of 1 or another value determined by the ratio between the selected values for R1 and R2. IOUT should track IIN across the range of input current values required by the design, however, even small differences in the physical characteristics of the BJTs, like for example, if they have different values of VBE and/or hfe then k1 will deviate from the desired value. This makes it more difficult to precisely control IOUT and therefore to define the behavior of the circuit it is biasing.
The differential amplifier
A differential amplifier features two BJT transistors Q1 and Q2 whose emitter terminals are connected together and to a current source. The output appears at the collector terminal of one (or both) of these devices depending on whether a single-ended or differential output voltage is required. When closely matched BJTs are used, common-mode amplification is extremely small and if these devices are thermally closely coupled, temperature changes only have the same effect as applying a slow-changing common-mode input voltage. The differential amplifier circuit shown in Figure 3 also uses two current mirrors – one for biasing and the other acting as the gain component for a single-ended output. Clearly, the considerations for current mirrors discussed previously also apply here.
The comparator
The comparator is similar to the differential amplifier in that they both have positive and negative input terminals. However, the output of a comparator is a digital signal (it has a low or a high state) whereas the output of a differential amplifier is analog. The transfer curve of a comparator should be steep and not have a wide input voltage range where the comparator exhibits linear behavior. Therefore, a high gain differential amplifier meets this requirement and can be used as a comparator. Integrated differential amplifiers and comparators normally only provide the positive output as a pin connection but in versions built using discrete BJTs, both output polarities can be made available for use. When using a comparator in a practical application, like turning a heater on and off in response to changing room temperature, it is recommended to include some degree of hysteresis in switching characteristics. This is to prevent the comparator output from oscillating when the input voltage (temperature) moves close to the reference value. This can be done by using a resistor to create a positive feedback loop between the output and one of the input terminals as shown in Figure 3.
Current sensing
Many applications like electric (EV) or mild-hybrid vehicles (HEV) for example, require the load current being drawn from a battery to be sensed close to the battery itself. The current must be converted to a voltage that is within the input range of an analog-to-digital converter (ADC) or a microcontroller and therefore it must have a small magnitude. This can be done using the current sensing circuit shown in Figure 5 where V1 represents the 48 V battery voltage and I1 is the load current drawn from it. Clearly, this circuit is based on two current mirrors – one pair of NPN BJTs and another pair of PNPs. A load is connected to the battery via a 50 mΩ current sensing resistor which means that for a maximum load current of 10 A, the input voltage to the sensing circuit will be 0.5 V. For the reasons discussed previously, it is desirable the BJTs in each current mirror pair to be as closely matched as possible, since any parameter mismatches (VBE or hfe) could cause an offset in the output voltage, which may need to be trimmed to ensure it remains within the input range of the ADC.
Nexperia’s matched pair BJTs outperform standard double transistors
For the above applications, it is recommended to use dual BJTs (two BJTs assembled in a single package). This ensures that the temperature of both dies will be almost identical because the two transistors are physically adjacent to each other. In additional, using matched devices guarantees that the electrical parameters of the transistor pair are almost identical, ensuring almost perfectly symmetrical behavior. Nexperia’s matched pair transistors eliminate the requirement for costly trimming in current mirror and differential amplifier applications. Compared to standard double transistors, they ensure accurate base-emitter voltage and current gain matching, and are fully internally isolated. The die pairs in each package are specifically harvested from the same wafer area to minimize the possibility of deviations in the manufacturing process. Assembling two dies inside a single-package also ensures they are thermally coupled. The final phase of testing ensures that the parameters fall within a narrow target window:
- hFE matching accuracy is between 2.2 - 10 % accuracy (depending on the device type)
- VBE matching <= 2 mV
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