CE And CC Amplifier

Common Emitter Amplifier

Sometimes called the grounded emitter, since the emitter capacitor connects the emitter to ground at ac frequencies. Since, as far as ac is concerned, the emitter is joined to ground, both input and output are connected to the emitter. Current gain is Ic/Ib and can be quite high, typically 50. Voltage gain is high, typically 250 Input impedance is medium, say 5K. Output impedance is medium, say 20k. The output is inverted with respect to the input. Its most common application is as a voltage amplifier. Common Collector Amplifier  The positive power supply rail is joined to the zero volts rail by C3. As far as ac is concerned, both rails are joined together. Therefore they, and the collector,  are common to both input and output. Since the emitter voltage follows the base voltage, it is also called the emitter follower. Current gain is Ie/Ib which is quite high, typically 50. Voltage gain is only 1 because of the undecoupled emitter. The input impedance is high, typically 500k, requiring only low power to drive it. The output impedance is low, typically 20 ohms. The output signal follows the input. There is no inversion. It is often used to match high impedances to low ones. It can be used to drive several high impedance loads. Common Base Amplifier  C3 connects the base to ground as far as ac is concerned. Therefore both input and output are connected to the base. (common base amplifier). Current gain is Ic/Ie which is less than 1. The voltage gain is high since it is Rc/Re. (Approximately the same current flows through them).  It is typically 250. The input impedance is low, typically 20 ohms. The output impedance is high, typically 1Megohm. The output signal is not inverted with respect the input. It is often used to match low impedance devices to high impedance ones. It is commonly used at VHF.

Junction Transistors

Junction Transistors

Junction transistors consist of two junctions made from N-type and P-type semiconductor materials and are called bipolar transistors (two polarities). They have three connections, emitter, base and collector. Transistor Operation  The forward biased base/emitter junction causes electrons to be attracted from the emitter area towards the base. Arriving in the base area, most of the negative electrons come under the influence of the more positive collector and are attracted by it. This is shown in the left hand drawing, where the base current plus collector current equals the emitter current. Alpha gain is collector current divided by emitter current, and is always less than 1. Beta gain is collector current divided by base current and can be a fairly high number. Therefore, causing a small base current to flow makes a much larger collector current flow. A small base current controls a large collector current. There is 0.6 volts across the base/emitter junction, when it is forward biased. (0.3 volts for germanium). Biasing a Transistor  Choose a general purpose transistor with a beta gain higher than 100. Decide on the collector current. The base bias voltage is be 1/3 of the supply voltage. The current through the base bias potential divider is to be 1/10 of the collector current. Calculate the two base resistor values, R1 and R2. The emitter voltage is 0.6 volts lower than the base voltage. The value of the emitter resistor R4 is the emitter voltage divided by the collector current. The value of the collector resistor R3 is the supply voltage divided by three times the collector current. The values of the capacitors depend upon the application. Study a few circuits. For common collector and common base some of the capacitors are connected differently.

Zener Diode Tutorial

Zener Diode

The Zener diode is operated in reverse bias mode (positive on its cathode). It relies on the reverse breakdown voltage occurring at a specified value. This value is printed on it.It has two main applications. 1. as a reference source, where the voltage across it is compared with another voltage. 2. as a voltage regulator, smoothing out any voltages variations occurring in the supply voltage across the load. When being used a voltage regulator, if the voltage across the load tries to rise then the Zener takes more current. The increase in current through the resistor causes an increase in voltage dropped across the resistor. This increase in voltage across the resistor causes the voltage across the load to remain at its correct value. In a similar manner, if the voltage across the load tries to fall, then the Zener takes less current. The current through the resistor and the voltage across the resistor both fall. The voltage across the load remains at its correct value.

Half Wave Rectifier

The voltage at point A does the opposite of that at point B. When A is increasing in a positive direction, B is increasing in a negative direction. It is rather like the two ends of a see-saw. During the first half cycle of the waveform shown on the left, A is positive and B is negative. The diode is forward biased and current flows around the circuit formed by the diode, the transformer winding and the load. Since the current through the load, and the voltage across the load are in the same proportions, then the voltage across the load is as shown in the right hand diagram, during the first half cycle. During the second half cycle, A and the anode are negative, B and  the cathode are positive. The diode is reverse biased and no current flows. This is indicated by the horizontal line in the right hand diagram. The diode only conducts on every other half cycle. There is one pulse for every cycle in. i.e 50 pulses per second (in the UK) The diode only conducts during half the cycle. Hence, HALF-WAVE RECTIFICATION. The rectified voltage is DC (it is always positive in value). However, it is not a steady DC but PULSATING DC. It needs to be smoothed before it becomes useful. If the diode is reversed then the output voltage is negative.

Introduction to Diodes

Diodes are polarised, which means that they must be inserted into the PCB the correct way round. This is because an electric current will only flow through them in one direction (like air will only flow one way through a tyre valve). Diodes have two connections, an anode and a cathode. The cathode is always identified by a dot, ring or some other mark. The pcb is often marked with a + sign for the cathode end. Barrier  At the junction, electrons fill holes so that there are no free holes or electrons there. The actual junction becomes an insulating layer. This barrier must be overcome before current can flow through the P-N junction. Diode Characteristic Curves  An electronic gate opens to let part of a signal through, and then shuts to reject the remainder. It's like separating sheep from goats, using a real gate. In the circuit, the cathodes of the diodes are more positive than the anodes. They are reverse biased and non conducting. The output of the circuit is isolated from the input. When the negative gating pulse comes along, the cathodes become more negative than the anodes. The diodes are forward biased and conduct. The output is connected to the input. During the duration of the gating pulse, the input signal appears at the output. as shown by the lowest waveform. The cathode end of the diode is usually marked in some manner. Diode Voltages  To forward bias a diode, the anode must be more positive than the cathode or LESS NEGATIVE. To reverse bias a diode, the anode must be less positive than the cathode or MORE NEGATIVE. A conducting diode has about 0.6 volts across if silicon, 0.3 volts if germanium. Forward Biased Junction  Bear in mind that like charges repel and unlikes attract. When a battery is connected as shown, the negative terminal pushes negative electrons towards the junction. The positive terminal pushes holes towards the junction. If the voltage is high enough then the barrier will be overcome and current will flow through the junction. There is a voltage across the diode. 0.6 for silicon, o.3 for germanium. The junction is said to be FORWARD BIASED. The P type is the anode of the diode, the N type the cathode, as shown by the diode symbol. The resistor limits the current to a safe level. Diodes come in all shapes and sizes. They are often marked with a type number. Detailed characteristics of a diode can be found by looking up the type number in a data book. If you know how to measure resistance with a meter then test some diodes. A good one has low resistance in one direction and high in the other. There are specialised types of diode available such as the zener and light emitting diode (LED).