Transistor
Contents
Multimeter transistor check
Bipolar transistors are constructed of a three-layer semiconductor sandwich either PNP or NPN. As such, transistors register as two diodes connected back-to-back when tested with a multimeter's resistance or diode check function as illustrated in Figure below. Low resistance readings on the base with the black negative (-) leads correspond to an N-type material in the base of a PNP transistor. On the symbol, the N-type material is "pointed" to by the arrow of the base-emitter junction, which is the base for this example. The P-type emitter corresponds to the other end of the arrow of the base-emitter junction, the emitter. The collector is very similar to the emitter, and is also a P-type material of the PN junction.
Here we are assuming the use of a multimeter with only a single continuity range (resistance) function to check the PN junctions. Some multimeters are equipped with two separate continuity check functions: resistance and “diode check,” each with its own purpose. If your meter has a designated “diode check” function, use that rather than the “resistance” range, and the meter will display the actual forward voltage of the PN junction and not just whether or not it conducts current.
Meter readings will be exactly opposite, of course, for an NPN transistor, with both PN junctions facing the other way. Low resistance readings with the red (+) lead on the base is the “opposite” condition for the NPN transistor.
If a multimeter with a “diode check” function is used in this test, it will be found that the emitter-base junction possesses a slightly greater forward voltage drop than the collector-base junction. This forward voltage difference is due to the disparity in doping concentration between the emitter and collector regions of the transistor: the emitter is a much more heavily doped piece of semiconductor material than the collector, causing its junction with the base to produce a higher forward voltage drop.
Knowing this, it becomes possible to determine which wire is which on an unmarked transistor. This is important because transistor packaging, unfortunately, is not standardized. All bipolar transistors have three wires, of course, but the positions of the three wires on the actual physical package are not arranged in any universal, standardized order.
Transistor Low Side Switch
Use this circuit when you wish to turn a load on and off with both a low voltage and a low current. Note that neither side of the load is grounded.
A low side switch is one which switches a circuit on and off at the ground or low side of the circuit. The advantage of a low side switch is that when using a transistor as the switch the voltage to drive the transistor is itself a low voltage. It is often the easy way to drive LEDS, motors, and other high current devices from such low power devices as PIC output ports. Low side switches are popular and there are many integrated circuits for them as well as this circuit.
Circuit with transistor:
Where
- LED is a low power LED
- R_LED is a current limiting resistor for the LED
- Q is a bipolar transistor
- R_1 is a current limiting resistor transistor base current
- VPLUS_VDD is the power supply for the LED
This circuit is sometimes called "grounded-emitter configuration". Note that this circuit can be realized with a bipolar transistor or fet. The bipolar transistor has a lower drive voltage ( usually well under 2 volts ) the fet can easily need 10 volts of drive -- use a logic level fet to reduce the drive voltage.
Some characteristics:
- Useful ( with simple circuits and common components ) for currents from a max of a few amps and voltages of 10's of volts.
- Can be very fast, into the Mega Hz.
- Can be very cheap at the low end.
- Small, simple.
- Some integrated circuit drivers like the are basically multiple transistor low side switches.
Transistor High Side Switch
Use this circuit when you wish to turn a load on and off with a voltage at a low current. Note that low side of the load is grounded. The voltage to turn on the switch is equal to the supply voltage ( or perhaps just a bit larger )
A high side switch is one which switches a circuit on and off at the supply voltage or high side of the circuit (the high side can be negative, it is a side away from ground). The advantage of a high side switch is that the load is grounded on one side. Compared to the low side switch it needs a higher voltage to drive it, but it also eliminates one resistor of that circuit. It the voltage to drive it is available it may be the circuit of choice. It is often the easy way to drive leds motors and other high current devices from such low power devices as PIC output ports.
Circuit with a transistor:
Where
- LED is a low power LED
- R_LED is a current limiting resistor for the LED
- Q is a bipolar transistor
- VPLUS_VDD is the power supply for the LED
The voltage at the collector of the circuit should fall to a fraction of a volt when the input is high. To compute the values in the circuit:
- Compute the value of R_LED using ohms law and the specifications for the LED.
- The current to drive the circuit is approximately the current to drive the load divided by the beta of the transistor.
No resistor is needed into the base of the transistor because as the load draws current the voltage at the base will rise and limit the base current. The input voltage should be about equal to VPLUS_VDD, high compared to that needed for the low side switch.
This high side switch usually requires the base voltage of Q to be VPLUS_VDD plus the turn-on voltage of the transistor to turn all the way on. Another approach to the high side switch that requires a lower turn-on voltage is to use a PNP transistor as the switch. The base of the PNP is pulled up to VPLUS_VDD and connected to the collector of a small signal NPN transistor, Q2. Q2's emitter is connected to ground and its base is connected to the input signal through a current limiting resistor -- now the problem is that a high voltage is required to turn the switch off.
Transistor Emitter Follower
Use this circuit when you have a signal of high impedance ( can supply only a little current ) that you want to connect to another circuit that draws a significant current. The circuit has no voltage gain, but because of the current gain it has a lot of power gain. It is frequently used in the final stage of an amplifier.
This circuit is a variation of the transistor high side switch. The difference is that we typically drive this circuit in a linear way ( all of the voltages between 0 and the supply voltage ) to make it a linear amplifier.
The emitter follower is also called a common collector circuit. The Emitter Follower is basically a high side switch, but when we call it an emitter follower we normally think of it as a linear ( analog ) amplifier, rather than as a switch.
Circuit:
Where
- R_LOAD represents the resistance of the load
- Q is a npn bipolar transistor
- VPLUS_VDD is the power supply for the Load
The current to drive the circuit is approximately the current to drive the load divided by the beta of the transistor. Use a Darlington connected transistor for a very high beta.
No resistor is needed into the base of the transistor because as the load draws current the voltage at the base will rise and limit the base current.
This circuit will only amplify positive voltages, using a pnp transistor you can amplify only negative voltages. Combine the two ( see push pull amplifier ) you can amplify both positive and negative voltages.
Variation of the circuit include:
- Use of coupling capacitors to amplify ac signals.
- Various other components to bias the transistor.
- Use an op amp buffer with voltage gain at the input, then the emitter follower for high current. In this way a few Milli volts with current on the order of pico amps can drive an output of several volts at an ampere or more.
