Transistors How do Amplifiers Work? All transistors are electrically controlled valves.

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1 Transistors Transistors are the magic of electronics. Transistors make possible things like radios, televisions, computers, the internet, etc. The reason transistors are so important is that they make amplifiers possible. When a radio wave strikes an antenna, it creates a small voltage in the antenna. The voltage is only a few microvolts, however. It must be amplified millions of times before it can be used to drive a speaker. When your voice strikes a microphone, the compression waves in the air make small voltages in the microphone. These voltages must be amplified thousands of times to be large enough to drive a speaker. Logic gates in computers could be made using just diodes, but the signal at the output would always be smaller than the signals at the inputs. Transistors are used in logic gates to amplify the signals back up so that the outputs can be as large as the inputs. Complex computations could not be done if logic gates did not contain transistor amplifiers. The signals would die out before the computation was finished. How do Amplifiers Work? How is amplification possible? It seems to violate conservation of energy. Amplification is possible because the energy at the output comes from a battery or power supply. The input controls a flow of energy coming from the power supply. All transistors are electrically controlled valves. Before we examine transistors, consider what can be done with a pneumatic valve controlling airflow from a compressor. Imagine a workman is using a jack hammer to break up a sidewalk. Jack hammers are powered by compressed air from a compressor. Suppose a valve is put in the air supply of the jack hammer, between the compressor and the jack hammer.

2 Suppose the valve is very well made so that it takes very little force to open and close it. A man can open and close it very easily with one hand. When he opens and closes the valve, he is controlling a large flow of energy to the jack hammer. A very small amount of force and energy is opening and closing the valve, and is controlling a large flow of energy to the jack hammer. There is amplification here. This is the principle by which all electronic amplifiers work. All transistors are electrical valves that can be opened or closed by a small input voltage or current. These valves control the flow of a much larger amount of power coming from a battery or power supply. Before transistors were invented, electronics was done using vacuum tubes. Vacuum tubes are also electronic valves. The British called vacuum tubes valves instead of tubes. Transistor Types There are three commonly used kinds of transistors, all of which are valves. 1. Bipolar transistors (npn & pnp) Fastest / Highest Amplification Used in some very high speed computer circuitry, and in many communication circuits 2. JFET (Junction Field Effect Transistors) Lowest Noise - For detecting very weak signals Used in television satellite dishes, and in communication receivers 3. MOSFET (MOS Field Effect Transistors) Smallest & Lowest Power Usage Used in most computer circuitry 99.9% of the transistors in computers are MOSFETs

3 Bipolar Transistor Basics We ll study all three common kinds of transistors, but we ll start with bipolar transistors. Bipolar transistors are the hardest to understand the physical mechanism by which they work (we won t try), but they are the easiest to do calculations for. They are the easiest to teach and understand if you don t ask what s going on inside of them. i Base controls i Collector. i Collector >> i Base (Assuming V CE > 0.2 volts) A typical transistor might behave as follows. 0 ma into the base allows 0 ma i C (The valve is closed).01 ma into the base allows 1 ma i C..02 ma into the base allows 2 ma i C. For a bipolar transistor, the collector current does not depend on the collector to emitter voltage as long as V CE is at least 0.2 volts! It acts like a dependent current source. The ratio i C / i B is called β. Different transistors have different β s. 20 < β < 200 for most transistors. A typical value is 100. Since the electron fluid is incompressible, all the charge that goes into the transistor must come out of it. i Emitter = i B + i C i C / i B = β, so i C = β i B.

4 Diodes in Transistors A transistor contains two junctions between p doped and n doped silicon: the Collector Base junction and the Base Emitter junction. A single pn junction is a diode. This is how diodes are made. The diode points from P to N. Memorize this.

5 You can think of a transistor as two diodes merged. If you make the middle p layer very thin & put a wire on it, you get a transistor. If it were really just two diodes, current would never flow into the collector! In fact, current β i B can flow into the collector even though it is against the diode.

6 Transistor Action allows the collector junction to leak when i B flows. It acts like the following circuit. This turns out to be a very good model for a bipolar npn transistor. If you replace each of the transistors in a circuit with this model and then analyze the circuit, you ll get the right answers. Any time you have a question about how a transistor acts, think about this model. The transistor acts just like this circuit! Do the transistor problem from in class practice problem set #10. I E & I B as a Function of V BE Suppose we know V BE. Is this enough information to calculate i E & i B? The answer is yes. Look at the transistor model. V B is the voltage at the top of the emitter diode, and V E is the voltage at the bottom of the emitter diode. Therefore, V BE = V B V E is the voltage across the emitter diode. If the model really acts like the transistor, then i E must equal the current that

7 flows through the diode when the diode voltage is V BE. i E as a function of V BE is just a diode curve. Now let s look at the base current. Does i B = i E? OF COURSE NOT! I B + i C = i E. In normal operation, the collector diode is reverse biased, so the current through the collector diode is zero in normal operation. Therefore, i C = βi B as long as the collector diode is reverse biased. i E = i B + βi B = (1+ β)i B. i E = (β + 1)i B as long as the collector diode is reverse biased. We can solve this equation for i B. i B = i E / (β + 1). If β = 100, then i B = i E / 101. To graph i B versus V BE, all we have to do is divide the currents by 101.

8 The graph of i B versus V BE also looks like a diode curve! It just has a smaller I 0. From the point of view of the input circuit, the transistor acts like a diode! This is an important simplification to keep in mind when analyzing transistor circuits.

9 Transistor Amplifiers Since i C = βi B, a bipolar transistor is a current amplifier. The collector current is a multiple of the base current. If we want to amplify voltages, we have to add resistors to the circuit. Assume a silicon transistor with β = 100. The diode cut in voltage is 0.7 volts for a silicon transistor. V OUT = (1K)i C = (1K)(βi B ) We can calculate i B by remembering that the input of the transistor acts like a diode. Assume V in is large enough that current is flowing into the base. (i.e., V in > 0.7 volt) This makes V BE = 0.7 volts. The voltage at the base of the transistor is therefore 0 volts volts = 0.7 volts. i B = i 10K = (V in 0.7volt)/(10K). V OUT = (1K)β(V in 0.7volt)/(10K). = (1K/10K) (100) (V in 0.7volt) V OUT = 10(V in 0.7volt) for V in > 0.7 volt

10 V OUT = 10(V in 0.7volt) for V in > 0.7 volt Except for the 0.7 volt term, V OUT = 10V in. What s i B when V in < 0.7 volts? It acts like a diode. i B = 0 when V in < 0.7 volts. Thus, V out = 0 volts when V in < 0.7 volts. The slope is called the AC voltage gain. A V = 10. Suppose we want to amplify a 10-5 volt signal from an antenna.

11 The slope is called the AC voltage gain. A V = 10. Suppose we want to amplify a 10-5 volt signal from an antenna. What V OUT do we get? We get 0 volts out, since V in < 0.7 volts!

12 The solution to this problem is to add a D.C. voltage, called a bias voltage, onto V in before amplifying it. V in = 1 volt + V signal The signal comes from the antenna through a small transformer. V OUT = 10(V in 0.7) = 10(V signal volt 0.7 volt) = 10V signal + 3 volts.

13 V OUT = 10V signal + 3 volts The AC part of the output is our signal. It is ten times as large as the input signal, so the AC voltage gain is 10. The D.C. part can be removed any time we want by passing the signal through a circuit using a capacitor or transformer. = =10 =10 When the input goes up & down 10-5 volts, the output goes up & down 10-4 volts. The AC gain is always the slope of the V out versus V in curve at the bias point.

14 AC and DC Components of Voltages & Currents When processing signals, one always breaks up the voltages into AC & DC components. The DC components are caused by the bias voltages. Total V OUT = DC V OUT + AC V OUT There is a special notation for this. Capital letters are used for DC voltages, and lower case letters are used for AC voltages. The total voltage is represented by a lower case letter with a capital subscript; it includes both. v OUT = V OUT + v out i B = I B + i b v C = V C + v c Total DC AC Total DC AC Total DC AC Everywhere in the circuit, voltages and currents can be split up into AC & DC components. Most amplifiers amplify only the AC components. Amplifiers with V OUT = V Collector Suppose we want to amplify a 10-5 volt (10 microvolt) signal from an antenna to 10 volts. We would need an amplification of A one transistor amplifier usually has a gain in the range of about We can get an amplification of 10 6 by connecting six amplifiers, each with an amplification of 10, as follows. This is called a cascade connection. The amplifiers are cascaded. Each amplifier makes the signal 10 times larger, and combined they make it 10 6 times larger.

15 In order to cascade amplifiers, the output of each amplifier must be connected to the input of the next. If we use the voltage across the collector resistor as V OUT, this will not work. Consider the circuit below. This connects the collector of the first transistor, Q1, to ground! This will make the collector voltage of the first transistor 0 volts. It will make V OUT for the first amplifier always equal to 10 volts 0 volts = 10 volts. The AC part of V OUT will be zero! While attempting to connect V OUT of the first amplifier to V in of the second, we have shorted out the transistor in the first amplifier! The usual solution to this problem is to take the output voltage across the transistor instead of the resistor. Now, V OUT = V CE = V C V E = V C 0 = V C. The collector voltage is the output voltage of the amplifier.

16 Let s graph V OUT versus V in for one transistor. V OUT = 10 volts V 1k = 10 volts (1k)i C = 10 volts (1k)βi B. i B = (V in 0.7 volt)/(10k) when V in > 0.7 volts. V OUT = 10 volts (1k)(100)(V in 0.7 volt)/(10k) when V in > 0.7 volt. V OUT = 10 volts- 10(V in 0.7 volt) when V in > 0.7 volt. When V in < 0.7 volt, i B = 0, so i C = 0. This makes V 1k = 0, so V OUT = 10 volts. V OUT = 10 volts when V in < 0.7 volt. V in = 1 volt gives V OUT = 7 volts.

17 Suppose V Bias = 1 volt. A V = slope at 1 volt = -10. = 10. When V in rises 10-5 volts, V OUT falls 10-4 volts, and vice versa.

18 The signal is amplified & turned upside down. It is inverted, or phase shifted 180 degrees. Notice that an amplifier with negative voltage gain still amplifies, but it turns the signal upside down. If you cascade two of these amplifiers, the second amplifier turns it back right side up. A cascade of an even number of amplifiers does not invert the signal; it just amplifies it.