Chapter 3-2 Semiconductor devices Transistors and Amplifiers-BJT Department of Mechanical Engineering

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1 MEMS1082 Chapter 3-2 Semiconductor devices Transistors and Amplifiers-BJT

2 Bipolar Transistor Construction

3 npn BJT

4 Transistor Structure

5 npn BJT I = I + E C I B V V BE CE = V = V B C V V E E Base-to-emitter pn junction is forward biased Base-to-collector pn junction is reverse biased Base is a very thin layer of p type region Emitter is more heavily n-doped than collector V > V > V C B E

6 Transistor as Current Amplifier The larger collector current I C is proportional to the base current I B according to the relationship I C =βi B, or more precisely it is proportional to the base-emitter voltage V BE. The smaller base current controls the larger collector current, achieving current amplification. The analogy to a valve is sometimes helpful. The smaller current in the base acts as a "valve", controlling the larger current from collector to emitter. A "signal" in the form of a variation in the base current is reproduced as a larger variation in the collector-to-emitter current, achieving an amplification of that signal. β is about 100 Amplification factor

7 pnp BJT Transistor Very thin

8 Transistor as Current Amplifier Constraints on Transistor Operation

9 BJT packages

10 Terminal characteristics of a typical BJT 0.2 V when saturation

11 Common emitter transistor circuit Transistor Characteristic Curves Controlled by collector circuit i c is proportional to i b 0.2 V when saturation

In the active region (some collector current, more than a few tenths of a volt above

12 Transistors Transistor Operation Cut off (no collector current), useful for switch operation. In the active region (some collector current, more than a few tenths of a volt above the emitter), useful for amplifier applications In saturation (collector a few tenths of a volt above emitter), large current useful for "switch on" applications.

13 Transistors Transistor Characteristic Curves

14 Transistors Transistor Characteristic Curves

15 Bipolar Transistor Switch V in =0.7V

Transistor Switch Example The base resistor is chosen small enough

In this example the mechanical switch is used to produce the base

In practice, any voltage on the base sufficient to drive the

There is no current to the base, so the transistor is in the cut

16 Transistor Switch Example The base resistor is chosen small enough so that the base current drives the transistor into saturation. In this example the mechanical switch is used to produce the base current to close the transistor switch to show the principles. In practice, any voltage on the base sufficient to drive the transistor to saturation will close the switch and light the bulb. There is no current to the base, so the transistor is in the cut off condition with no collector current. All the voltage drop is across the transistor.

Transistor Switch Example NPN Common Emitter Switch For switching currents less than an

Instead of a mechanical switch in the base circuit, an op-amp could be used.

The resistor R B must be small enough to drive the transistor to saturation so that most of

In a configuration where the output is taken below the load resistor, this kind of switch can

17 Transistor Switch Example NPN Common Emitter Switch For switching currents less than an ampere, the transistor switch can be used. Instead of a mechanical switch in the base circuit, an op-amp could be used. When the switch is open, no current flows in the base so the collector current is cut off. The resistor R B must be small enough to drive the transistor to saturation so that most of the voltage V cc appears across the load Rc. In a configuration where the output is taken below the load resistor, this kind of switch can function as an inverting buffer in digital circuits. For equal resistors, it has a gain of -1, and is used in digital circuits as an inverting buffer. A transistor switch with collector resistor can serve as an inverting buffer.

18 Transistor Switch Example Op-Amp Switching If an op-amp is used instead of a mechanical switch to operate a transistor switch. The diode is used for protection of the base-emitter junction in case the op-amp swings to its negative supply voltage.

19 LED Switch O V or 5V

20 Darlington Pair The emitter current of TR1 is the base current of TR2. A change in base current of TR1 can give a change 100 times larger in its emitter current. A change in the base current of TR2 has a similar effect on its emitter current. Therefore there is an overall amplification of 100 x 100 = times. This circuit is sometimes called the Super Alpha Pair. It is often used as a power output stage. The two transistors can come in the same package.

Phototransistor and Optoisolator An electronic device designed to transfer electrical signals by utilizing light waves to provide coupling with

21 Phototransistor and Optoisolator An electronic device designed to transfer electrical signals by utilizing light waves to provide coupling with electrical isolation between its input and output Planar (top) and silicone dome (bottom) layouts - cross-section through a standard dual in-line package

22 Guarantee a transistor in saturation Q:

23 Common emitter npn BJT

24 Common emitter npn BJT current amplifier The Common Emitter Amplifier Circuit This type of configuration is the most commonly used circuit for transistor based amplifiers and which represents the "normal" method of bipolar transistor connection.

25 Common emitter npn BJT current amplifier I = I + E C I B V > V > V C B E The collector current I C is proportional to the base current I B according to the relationship I C =βi B β has a value between 20 and 200 for most general purpose transistors

26 Common emitter npn BJT current amplifier

27 BJT current amplifier

28 Common base pnp BJT voltage amplifier Common base The BASE connection is common to both the input signal AND the output signal with the input signal being applied between the base and the emitter terminals. The corresponding output signal is taken from between the base and the collector terminals with the base terminal grounded. The input current flowing into the emitter is quite large as its the sum of both the base current and collector current respectively therefore, the collector current output is less than the emitter current input resulting in a current gain for this type of circuit of "1" (unity) or less, in other words the common base configuration "attenuates" the input signal.

29 pnp BJT voltage amplifier This type of amplifier configuration is a non-inverting voltage amplifier circuit, in that the signal voltages V in and V out are in-phase. This type of transistor arrangement is not very common due to its unusually high voltage gain characteristics due to its high ratio of output to input resistance or more importantly "load" resistance (R L ) to "input" resistance (R in ) giving it a value of "Resistance Gain". Then the voltage gain (Av for a common base configuration

Emitter Follower Circuit a large Current gain This type of configuration is commonly known as a Voltage Follower or Emitter Follower circuit.

30 Emitter Follower Circuit a large Current gain This type of configuration is commonly known as a Voltage Follower or Emitter Follower circuit. This type of bipolar transistor configuration is a non-inverting circuit in that the signal voltages of V in and V out are in-phase. It has a voltage gain that is always less than "1" (unity).

31 Emitter Follower Switch NPN Emitter Follower Switch The emitter follower can also be used for switching. The resistor R B must be small enough to drive the transistor to saturation so that most of the voltage V cc appears across the load.

32 Angular Position of a Robotic Scanner Photo-interrupter

Lecture 3: Transistors

Lecture 3: Transistors Now that we know about diodes, let s put two of them together, as follows: collector base emitter n p n moderately doped lightly doped, and very thin heavily doped At first glance,