The Bipolar Transistor as a Switch

A switch is a device that is used to 'open' or 'close' a circuit. Opening a circuit means creating a break in the circuit, preventing current flow and thus, turning it 'off'. Closing a circuit, on the other, means completing the circuit path, thereby allowing current to flow around it and thus, turning it 'on'.

The bipolar transistor, whether NPN or PNP, may be used as a switch. Recall that the bipolar transistor has three regions of operation: the cut-off region, the linear or active region, and the saturation region. When used as a switch, the bipolar transistor is operated in the cut-off region (the region wherein the transistor is not conducting, and therefore makes the circuit 'open') and saturation region (the region wherein the transistor is in full conduction, thereby closing the circuit). The bipolar transistor is a good switch because of its large transconductance Gm, with Gm = Ic/Vbe where Ic is the collector-to-emitter (output) current and Vbe is the base-emitter (input) voltage. Its high Gm allows large collector-to-emitter currents to be easily achieved if sufficient excitation is applied at the base. To illustrate this, the simplest way to use an NPN bipolar transistor as a switch is to insert the load between the positive supply and its collector, with the emitter terminal grounded (as shown in Figure 1). Applying no voltage at the base of the transistor will put it in the cut-off region, preventing current from flowing through it and through the load, which is a resistor in this example. In this state, the load is 'off'.

Figure 1. A Simple Switch Using an NPN Transistor Applying enough voltage at the base of the transistor will cause it to saturate and become fully conductive, effectively pulling the collector of the transistor to near ground. This causes a collectorto-emitter current to flow through the load that's limited only by the impedance of the load. In this state, the load is 'on'. One limitation of this simple design is that the switch-off time of the transistor is slower than its switch-on time if the load is a resistor. This is because of the stray capacitance across the collector of the transistor and ground, which needs to charge through the load resistor during switch-off. On the other hand, this stray capacitance is easily discharged to ground by the large collector current flow when the transistor is switched on. There are, of course, other better designs for using the bipolar transistor as a switch.

Because a transistor's collector current is proportionally limited by its base current, it can be used as a sort of current-controlled switch. A relatively small flow of electrons sent through the base of the transistor has the ability to exert control over a much larger flow of electrons through the collector. Suppose we had a lamp that we wanted to turn on and off with a switch. Such a circuit would be extremely simple as in Figure below(a). For the sake of illustration, let's insert a transistor in place of the switch to show how it can control the flow of electrons through the lamp. Remember that the controlled current through a transistor must go between collector and emitter. Since it is the current through the lamp that we want to control, we must position the collector and emitter of our transistor where the two contacts of the switch were. We must also make sure that the lamp's current will move against the direction of the emitter arrow symbol to ensure that the transistor's junction bias will be correct as in Figure below(b).

(a) mechanical switch, (b) NPN transistor switch, (c) PNP transistor switch. A PNP transistor could also have been chosen for the job. Its application is shown in Figure above(c). The choice between NPN and PNP is really arbitrary. All that matters is that the proper current directions are maintained for the sake of correct junction biasing (electron flow going against the transistor symbol's arrow). Going back to the NPN transistor in our example circuit, we are faced with the need to add something more so that we can have base current. Without a connection to the base wire of the transistor, base current will be zero, and the transistor cannot turn on, resulting in a lamp that is always off. Remember that for an NPN transistor, base current must consist of electrons flowing from emitter to base (against the emitter arrow symbol, just like the lamp current). Perhaps the simplest thing to do would be to connect a switch between the base and collector wires of the transistor as in Figure below (a).

Transistor: (a) cutoff, lamp off; (b) saturated, lamp on. If the switch is open as in (Figure above (a), the base wire of the transistor will be left floating (not connected to anything) and there will be no current through it. In this state, the transistor is said to be cutoff. If the switch is closed as in (Figure above (b), however, electrons will be able to flow from the emitter through to the base of the transistor, through the switch and up to the left side of the lamp, back to the positive side of the battery. This base current will enable a much larger flow of electrons from the emitter through to the collector, thus lighting up the lamp. In this state of maximum circuit current, the transistor is said to be saturated. Of course, it may seem pointless to use a transistor in this capacity to control the lamp. After all, we're still using a switch in the circuit, aren't we? If we're still using a switch to control the lamp -- if only indirectly -- then what's the point of having a transistor to control the current? Why not just go back to our original circuit and use the switch directly to control the lamp current? Two points can be made here, actually. First is the fact that when used in this manner, the switch contacts need only handle what little base current is necessary to turn the transistor on; the transistor itself handles most of the lamp's current. This may be an important advantage if the switch has a low current rating: a small switch may be used to control a relatively highcurrent load. More important, the current-controlling behavior of the transistor enables us to use something completely different to turn the lamp on or off. Consider Figure below, where a pair of solar cells provides 1 V to overcome the 0.7 VBE of the transistor to cause base current flow, which in turn controls the lamp.

Solar cell serves as light sensor. Or, we could use a thermocouple (many connected in series) to provide the necessary base current to turn the transistor on in Figure below.

A single thermocouple provides 10s of mV. Many in series could produce in excess of the 0.7 V transistor VBE to cause base current flow and consequent collector current to the lamp. Even a microphone (Figure below) with enough voltage and current (from an amplifier) output could turn the transistor on, provided its output is rectified from AC to DC so that the emitterbase PN junction within the transistor will always be forward-biased:

Amplified microphone signal is rectified to DC bias the base of the transistor providing a larger collector current. The point should be quite apparent by now: any sufficient source of DC current may be used to turn the transistor on, and that source of current only need be a fraction of the current needed to energize the lamp. Here we see the transistor functioning not only as a switch, but as a true amplifier: using a relatively low-power signal to control a relatively large amount of power. Please note that the actual power for lighting up the lamp comes from the battery to the right of the schematic. It is not as though the small signal current from the solar cell, thermocouple, or microphone is being magically transformed into a greater amount of power. Rather, those small power sources are simply controlling the battery's power to light up the lamp.

REVIEW: Transistors may be used as switching elements to control DC power to a load. The switched (controlled) current goes between emitter and collector; the controlling current goes between emitter and base.

When a transistor has zero current through it, it is said to be in a state of cutoff (fully nonconducting). When a transistor has maximum current through it, it is said to be in a state of saturation (fully conducting).

The Transistor as a Switch

When used as an AC signal amplifier, the transistors Base biasing voltage is applied so that it always operates within its "active" region, that is the linear part of the output characteristics curves are used. However, both the NPN & PNP type bipolar transistors can be made to operate as an "ON/OFF" type solid state switch by biasing its Base differently to that of an amplifier. Solid state switches are one of the main applications of transistors. Transistor switches are used for controlling high power devices such as motors, solenoids or lamps, but they can also be used in digital electronics and logic gate circuits. If the circuit uses the Bipolar Transistor as a Switch, then the biasing of the transistor, either NPN or PNP is arranged to operate at the sides of the V-I characteristics curves we have seen previously. The areas of operation for a transistor switch are known as the Saturation Region and the Cut-off Region. This means then that we can ignore the operating Q-point biasing and voltage divider circuitry required for amplification, and use the transistor as a switch by driving it back and forth between "fully-OFF" (cut-off region) and "fully-ON" (saturation region) as shown below.

Operating Regions

The pink shaded area at the bottom of the curves represents the "Cut-off" region while the blue area to the left represents the "Saturation" region of the transistor. Both these transistor regions are defined as:

1. Cut-off Region
Here the operating conditions of the transistor are zero input base current ( IB ), zero output collector current ( IC ) and maximum collector voltage ( VCE ) which results in a large depletion layer and no current flowing through the device. Therefore the transistor is switched "Fully-OFF".

Cut-off Characteristics

The input and Base are grounded (0v) Base-Emitter voltage VBE < 0.7V Base-Emitter junction is reverse biased Base-Collector junction is reverse biased Transistor is "fully-OFF" (Cut-off region) No Collector current flows ( IC = 0 )

VOUT = VCE = VCC = "1"

Transistor operates as an "open switch"

Then we can define the "cut-off region" or "OFF mode" of a bipolar transistor switch as being, both junctions reverse biased, IB < 0.7V and IC = 0. For a PNP transistor, the Emitter potential must be negative with respect to the Base.

2. Saturation Region
Here the transistor will be biased so that the maximum amount of base current is applied, resulting in maximum collector current resulting in the minimum collector emitter voltage drop which results in the depletion layer being as small as possible and maximum current flowing through the transistor. Therefore the transistor is switched "FullyON".

Saturation Characteristics

The input and Base are connected to VCC Base-Emitter voltage VBE > 0.7V Base-Emitter junction is forward biased Base-Collector junction is forward biased Transistor is "fully-ON" (saturation region) Max Collector current flows (IC = Vcc/RL)

VCE = 0 (ideal saturation) VOUT = VCE = "0"

Transistor operates as a "closed switch"

Then we can define the "saturation region" or "ON mode" of a bipolar transistor switch as being, both junctions forward biased, IB > 0.7V and IC = Maximum. For a PNP transistor, the Emitter potential must be positive with respect to the Base.

Then the transistor operates as a "single-pole single-throw" (SPST) solid state switch. With a zero signal applied to the Base of the transistor it turns "OFF" acting like an open switch and zero collector current flows. With a positive signal applied to the Base of the transistor it turns "ON" acting like a closed switch and maximum circuit current flows through the device.

An example of an NPN Transistor as a switch being used to operate a relay is given below. With inductive loads such as relays or solenoids a flywheel diode is placed across the load to dissipate the back EMF generated by the inductive load when the transistor switches "OFF" and so protect the transistor from damage. If the load is of a very high current or voltage nature, such as motors, heaters etc, then the load current can be controlled via a suitable relay as shown.

Basic NPN Transistor Switching Circuit

The circuit resembles that of the Common Emitter circuit we looked at in the previous tutorials. The difference this time is that to operate the transistor as a switch the transistor needs to be turned either fully "OFF" (cut-off) or fully "ON" (saturated). An ideal transistor switch would have infinite circuit resistance between the Collector and Emitter when turned "fully-OFF" resulting in zero current flowing through it and zero resistance between the Collector and Emitter when turned "fully-ON", resulting in maximum current flow. In practice when the transistor is turned "OFF", small leakage currents flow through the transistor and when fully "ON" the device has a low resistance value causing a small saturation voltage (VCE) across it. Even though the transistor is not a perfect switch, in both the cut-off and saturation regions the power dissipated by the transistor is at its minimum.

In order for the Base current to flow, the Base input terminal must be made more positive than the Emitter by increasing it above the 0.7 volts needed for a silicon device. By varying this Base-Emitter voltage VBE, the Base current is also altered and which in turn controls the amount of Collector current flowing through the transistor as previously discussed. When maximum Collector current flows the transistor is said to be Saturated. The value of the Base resistor determines how much input voltage is required and corresponding Base current to switch the transistor fully "ON".

Example No1
Using the transistor values from the previous tutorials of: = 200, Ic = 4mA and Ib = 20uA, find the value of the Base resistor (Rb) required to switch the load "ON" when the input terminal voltage exceeds 2.5v.

The next lowest preferred value is: 82k, this guarantees the transistor switch is always saturated.

Example No2
Again using the same values, find the minimum Base current required to turn the transistor "fully-ON" (saturated) for a load that requires 200mA of current when the input voltage is increased to 5.0V. Also calculate the new value of

Rb.
transistor Base current:

transistor Base resistance:

Transistor switches are used for a wide variety of applications such as interfacing large current or high voltage devices like motors, relays or lamps to low voltage digital logic IC's or gates like AND gates or OR gates. Here, the output from a digital logic gate is only +5v but the device to be controlled may require a 12 or even 24 volts supply. Or the load such as a DC Motor may need to have its speed controlled using a series of pulses (Pulse Width Modulation). transistor switches will allow us to do this faster and more easily than with conventional mechanical switches.

Digital Logic Transistor Switch

The base resistor, Rb is required to limit the output current from the logic gate.

PNP Transistor Switch

We can also use PNP transistors as switches, the difference this time is that the load is connected to ground (0v) and the PNP transistor switches power to it. To turn the PNP transistor as a switch "ON" the Base terminal is connected to ground or zero volts (LOW) as shown.

PNP Transistor Switching Circuit

The equations for calculating the Base resistance, Collector current and voltages are exactly the same as for the previous NPN transistor switch. The difference this time is that we are switching power with a PNP transistor (sourcing current) instead of switching ground with an NPN transistor (sinking current).

Darlington Transistor Switch

Sometimes the DC current gain of the bipolar transistor is too low to directly switch the load current or voltage, so multiple switching transistors are used. Here, one small input transistor is used to switch "ON" or "OFF" a much larger current handling output transistor. To maximise the signal gain, the two transistors are connected in a "Complementary Gain Compounding Configuration" or what is more commonly called a "Darlington Configuration" were the amplification factor is the product of the two individual transistors. Darlington Transistors simply contain two individual bipolar NPN or PNP type transistors connected together so that the current gain of the first transistor is multiplied with that of the current gain of the second transistor to produce a device which acts like a single transistor with a very high current gain for a much smaller Base current. The overall current gain Beta () or Hfe value of a Darlington device is the product of the two individual gains of the transistors and is given as:

So Darlington Transistors with very high

values and high Collector currents are possible compared to a single

transistor switch. For example, if the first input transistor has a current gain of 100 and the second switching transistor has a current gain of 50 then the total current gain will be 100 x 50 = 5000. An example of the two basic types of Darlington transistor are given below.

Darlington Transistor Configurations

The above NPN Darlington transistor switch configuration shows the Collectors of the two transistors connected together with the Emitter of the first transistor connected to the Base of the second transistor therefore, the Emitter current of the first transistor becomes the Base current of the second transistor. The first or "input" transistor receives an input signal, amplifies it and uses it to drive the second or "output" transistors which amplifies it again resulting in a very high current gain. As well as its high increased current and voltage switching capabilities, another advantage of a Darlington transistor switch is in its high switching speeds making them ideal for use in inverter circuits and DC motor or stepper motor control applications.

One difference to consider when using Darlington transistors over the conventional single bipolar types when using the transistor as a switch is that the Base-Emitter input voltage ( VBE ) needs to be higher at approx 1.4v for silicon devices, due to the series connection of the two PN junctions.

Transistor as a Switch Summary

Then to summarise when using a Transistor as a Switch.

Transistor switches can be used to switch and control lamps, relays or even motors. When using the bipolar transistor as a switch they must be either "fully-OFF" or "fully-OFF". Transistors that are fully "ON" are said to be in their Saturation region. Transistors that are fully "OFF" are said to be in their Cut-off region. When using the transistor as a switch, a small Base current controls a much larger Collector load current. When using transistors to switch inductive loads such as relays and solenoids, a "Flywheel Diode" is used. When large currents or voltages need to be controlled, Darlington Transistors can be used. In the next tutorial about Transistors, we will look at the operation of the junction field effect transistor known commonly as an JFET. We will also plot the output characteristics curves commonly associated with JFET amplifier circuits as a function of Source voltage to Gate voltage.