What is a Complementary Transistor Pair?
Complementary Transistors are a set, or “pair” of bipolar transistors that consist of one NPN and one PNP transistor which are designed to work together as part of a symmetrical circuit configuration.
For bipolar transistors, the term complementary simply means a “matching pair”. With one being an NPN-type and the other being a PNP-type. Even though they are both individual discrete bipolar junction transistors, it is not too difficult to produce complementary transistors within one single semiconductor package that are electrically isolated from each other.
For example, the surface-mount BC847PN which is a general-purpose matched pair featuring an NPN and PNP transistor within a 6-pin package and based on the popular BC847/BC857 series.
Matched Transistor Pairs
Then the complementary nature of two matching NPN and PNP transistors means that they have almost identical electrical characteristics, rather than just similar ones, with the exception of the current and voltage signs. Since an NPN transistor requires a positive (+ve) drive, while a PNP transistor requires a negative (-ve) drive.
The relationship between the two transistors is how they use their voltages and the direction of current flow allowing matched transistor pairs to operate symmetrically during both positive and negative voltage swings.
Note that the expression “complementary transistors” can also be extended to Junction-FETs (JFETs), MOSFETs and CMOS based technology. Where one device is an N-channel (nMOS), and its complement is the P-channel (pMOS). As such they share similar structures and switching performance for use in amplification and switching circuits.
But first, let us remind ourselves of the switching characteristics of a single NPN and PNP bipolar transistor.
The Bipolar Junction Transistor
We know from our Bipolar Junction Transistor tutorial that the BJT is a three-element (emitter, base, and collector) device constructed from alternating layers of n-type and p-type semiconductor materials joined together. Then depending on their internal structure, bipolar transistors are available in NPN and PNP types as shown.
The NPN Bipolar Transistor Configuration
The bipolar NPN Transistor consists of one P-type semiconductor layer sandwiched between two outer N-type layers.
The transistor “turns ON” and conducts when a positive voltage higher than the emitter terminal is applied to its base terminal. Then for an NPN transistor, current must flow from the base into the emitter.
The PNP Bipolar Transistor Configuration
The PNP Transistor is the opposite consisting of an inner N-type semiconductor layer inbetween two outer P-type layers as shown.
The transistor “turns ON” when the base voltage is lower (more negative) than the emitter terminal voltage allowing a small control current to flow from the emitter into the base. Thus the base must be held at a lower voltage level compared to the emitter.
What are Complementary Matched Pair Transistors?
Complementary Transistors Pairs consist of one NPN and one PNP transistor which are designed to work together by utilising the distinct characteristics of both the NPN and PNP transistors previously discussed.
But in order to work together in a circuit as a complementary transistor pair, the two bipolar transistors must have matching electrical characteristics (such as gain and power rating) allowing them to operate symmetrically but with opposite polarities.
In a complementary transistor configuration, the NPN transistor conducts when a positive voltage is applied to its base, while the PNP transistor conducts when a negative voltage is applied to its base. As such, they are ideal for use in push-pull Class-B Amplifier circuits because they can handle both halves of an AC input signal.
Class-B amplifiers get their “push-pull” name from the fact that the NPN transistor “pushes” current into the load during the positive half-cycle while the PNP transistor “pulls” current from the load during the negative half-cycle.
But as well as using bipolar transistors as amplifiers by biasing them in their linear region of the characteristics curve, we can also use bipolars as solid-state ON/OFF switching device. For example, a transistor is used as a switch to turn an LED “ON” and “OFF”.
Using The Bipolar Transistor As A Switch
Transistor Biasing establishes the DC operating point (Q-point) for the proper linear operation of a bipolar transistor as an amplifier of sinusoidal signals. Biasing the transistor in the active (or linear) region gives it current gain (Beta, β).
However, when used as an electronic switch the type or shape of the input–output characteristic of the transistor is of little importance since the bipolar junction transistor (BJT) is normally operated alternately between its Cut-off and Saturation regions. That is, the transistor is operated between two states usually termed “ON” and “OFF”.
This two-state ON/OFF switching action allows them to be used in power switching circuits such as relay or solenoid drivers, DC motor and stepping motor control. As well as other types of Output Interfacing Circuits.
Then a transistor can be used as an ON/OFF solid-state switch simply by driving it back and forth between its saturation and cut-off regions as shown.
The NPN and PNP Transistor Switch
Here in this simple example of Using The Bipolar Transistor as a Switch. When the base input voltage (Vb) to the NPN transistor is zero (or negative), the transistors base-emitter junction is reverse-biased, so the NPN transistor is cut-off and no current flows in collector (Ic = 0). As a result, there is no voltage drop across the LED so it remains OFF. Therefore this condition is similar to that of an open switch.
However, when the base input voltage (Vb) to the PNP transistor is zero (or negative), the transistors base-emitter junction is forward-biased, so the PNP transistor becomes saturated and load current flows in collector (Ic = Imax). As a result the LED is switched ON and this condition is similar to that of a closed switch.
Now when the input base voltage (Vb) is positive enough (greater than 0.7V) the base-emitter junction becomes forward-biased. This causes the NPN transistor to saturate and current flows in the collector (Ic = Imax). As a result the LED is switched ON. This condition is similar to that of a closed switch.
Again the reverse is true for the PNP transistor. When the base input voltage (Vb) to the PNP transistor is positive (above zero), the transistors base-emitter junction is reverse-biased, so the PNP transistor is cut-off and no current flows in collector (Ic = 0). As a result the LED is OFF with this condition being similar to that of an open switch.
Then we can see that a bipolar transistor (or FET) can behave as a solid-state switch under proper biasing conditions. That is, if the input base voltages are enough negative or positive, the transistor will be driven between cut-off and saturation. With the switching actions of the NPN transistor and PNP transistor being opposite, or complementary to each other.
Complementary Transistors Switching Application
Now that we have seen how each type of transistor can be controlled, we can connect them together to form Complementary NPN/PNP Transistor Pairs. However, simply connecting any NPN and PNP together is not enough, we require “matched” bipolar transistor pairs.
Matching means that both transistors have nearly identical characteristics for: Current Gain (β or hFE), its Saturation Current (ISAT), Base-Emitter Voltage, VBE, etc. among others. Also, complementary transistor pairs provide symmetrical performance for efficient power usage and ON/OFF switching control.
Thus if the transistors are not matched, one may conduct more than the other resulting in it getting hotter as it draws more current with eventual failure. This also applies to JFETs and MOSFET technology, where one device is an N-channel, and the other is a P-channel.
Then connecting together the previous NPN and PNP switching circuits, we can create a complementary symmetry transistor pair as shown below.
Complementary Transistors Matched Pair
Q1 is the NPN transistor and Q2 is the PNP transistor with both switching transistors Q1 and Q2 assumed to have identical characteristics.
When switch S1 is up at position “A”, the base terminals of the two transistors are connected directly to Vcc via their respective base resistors, RB. Thus with a positive voltage applied to each base terminal, transistor Q1 will be forward biased in to conduction while the base-emitter junction of Q2 is reverse biased so will remain OFF.
With transistor Q1 fully-ON and supplying current from the positive power supply, Vcc to Vout. The bottom light emitting diode (LED2) will illuminate as it is connected between the output terminal and ground, while LED1 is OFF. Thus transistor Q1 “Sources” (supplies) current to the LED.
Now if we change switch S1 to position “B”, the base terminals of the two transistors are connected directly to ground (0V) via their base resistors, RB. This zero or negative voltage biases transistor Q2 into conduction while Q1 remains OFF since the voltage is below 0.7V.
Then with transistor Q2 fully-ON and sinking current from Vout to ground. The top light emitting diode (LED1) will illuminate as it is connected between Vcc and Vout, while LED2 is OFF. Thus transistor Q2 “Sinks” (absorbs) current from the LED.
Then the two complementary transistors Q1 and Q2 have the ability to either “Sink” or “Source” a load current of up to their maximum rated value, which in this example is sufficient to directly drive the LEDs.
Complementary Transistor H-bridge
So now we know that a complementary transistor pair can be used to both sink and source a current depending on a HIGH or LOW base drive voltage, what else can we use it for.
Well one interesting application is that of an H-bridge Circuit which allows for the bidirectional rotation of a small DC motor simply by changing the voltage applied to the bases of the four transistors.
Basic H-bridge Motor Control Topology
The H-bridge circuit gets its named because the basic configuration of four switching elements resembles that of the letter “H” when the circuit is drawn with the motor load positioned inside on the centre bar.
So by controlling the individual “switches”, the current through the center load can be made to vary in both direction and intensity.
This H-shape configuration is a simple and effective way to achieve bidirectional motor control (or any load) from a fixed DC power supply.
Then we can use our complementary transistor circuit within this H-bridge configuration of switches to control the rotational direction of a motor.
This is achieved by turning the switching elements “ON” or “OFF” in pairs (A1 – A2, and B1 – B2) that are diagonally opposite each other in complementary pairs since one is an NPN device, and the other is a PNP device.
Let’s be clear here, there are many different ways to create a H-bridge circuit using discrete transistors to control the rotational direction of a small DC motor. As such there are dedicated monolithic H-bridge ICs available, like the L293D.
The L293D is popular for driving small DC motors up to 36 volts and 1 Ampere per motor winding. Or the larger L298N which can handle up to 2 Amperes per motor winding.
But since this tutorial is about Complementary NPN/PNP Transistor Pairs then lets use them to build a very simple H-bridge circuit as shown.
Complementary Transistors for H-bridge Motor Control
So how does it work. With the switch (S1) up at position “A”, The base terminal of transistor pairs A1 and A2 are connected to +Vcc. Thus being NPN transistors they are biased into conduction allowing motor current to flow from left to right rotating it in one direction.
PNP transistor pairs B1 and B2 also have a +Vcc voltage level at their base terminals and are therefore cut-off as both their base and collector terminals are more positive than their emitters.
Likewise, with the switch down at position “B”, the base terminals of NPN transistor pairs A1 and A2 are connected to 0 volts (GND), so turn OFF. However, PNP transistor pairs B1 and B2 are also connected to 0 volts (GND) biasing them into saturation.
So with the NPN transistors A1 and A2 in cut-off and the PNP transistors B1 and B2 biased into conduction. This allows the motor current to flow in the opposite direction from right to left rotating the motor in the other direction.
Then the direction of the motor is controlled by switching the transistors “ON” or “OFF” in their “diagonal matched pairs” results in bidirectional control. Also, these bipolar transistor switches could be replaced with discrete N-channel and P-channel power E-MOSFETs which offer faster switching speeds, and more resemble the operation of an ideal switch.
Note that while this complementary transistor H-bridge circuit is cheap and easy to build, it does have its disadvantages. The main one is the efficiency of the DC motor, and the ohmic size of the four base biasing resistors since you want sufficient base drive of the transistors to drive them deep into saturation without drawing too much current and overheating.
Clearly, the type of complementary NPN/PNP switching transistors used to operate between saturation and cut-off will depend on the power rating of the DC motor load. But complementary transistor pairs are available in a range of current and power ratings as shown in the complementary transistor pairs list below.
Complementary NPN and PNP Transistor Pairs List
| NPN | PNP | VCE | IC(max) | Pd |
| 2N3904 | 2N3906 | 40v | 200mA | 625mW |
| 2N2222 | 2N2907 | 30v | 800mA | 800mW |
| BD135 | BD136 | 45v | 1.5A | 12.5W |
| BD179 | BD180 | 80v | 3A | 30W |
| BD243C | BD244C | 100v | 6A | 65W |
| BC107 | BC177 | 45v | 200mA | 600mW |
| BC140 | BC160 | 40v | 1A | 800mW |
| BC447 | BC448 | 80v | 300mA | 625mW |
| BC548 | BC558 | 30v | 100mA | 625mW |
| TIP29C | TIP30C | 100v | 1A | 30W |
| TIP41A | TIP42A | 60v | 6A | 65W |
| TIP35C | TIP36C | 100v | 25A | 125W |
| TIP3055 | TIP2955 | 60v | 15A | 80W |
One of the most common complementary pairs example is the small-signal 2N3904 (NPN) transistor and its complement the 2N3906 (PNP) transistor. Another classic example is the BC548 (NPN) and PNP BC558 (PNP) pair useful in small signal amplification and relay output switching circuits.
The 2N3055 and the MJ2955 are 15 Ampere complementary power transistors designed for general purpose motor switching and high-power amplifier applications.
Complementary Transistors Tutorial Summary
We have seen here that Complementary Transistors are bipolar PNP and NPN transistors with almost identical electrical characteristics, such as gain, voltage and power rating.
However, if properly matched, the beauty of NPN/PNP transistor pairs lies in their symmetry while operating with opposite polarities. This symmetry allows for the creation of efficient output stages in audio amplifiers, switching circuits and motor controllers.
The most common application of matched transistor pairs is in Class-B Push-Pull Amplifiers and Class AB Stages where the NPN transistor handles the the positive half-cycle of the input signal. While the PNP transistor handles the negative half-cycle allowing for a more linear amplification.
We have also seen that complementary transistors can be used to construct switching circuits for efficient ON/OFF control of LEDs, motors and many power electronics applications.
However, using complementary transistors pairs instead of a single transistor switch can make the circuit more complex in its design leading to higher costs and the need for precise matching of transistor pairs.
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Curated by Jesif Ahmed.
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