Explain Analog Circuit With Diagram

What Is Analog Circuit, Analog Circuit , What is Analog, How Analog Circuit works?

Hello friends...this post is continued part of my last post....:)

C. Single-stage amplifiers

The common emitter configuration can be modified to produce an output signal that is linearly proportional to the input, a function known as amplification. The trick is to supply a constant base current, called a bias current, so that the transistor is partially turned on. The signal current is then superimposed on the constant bias current. The resulting collector current will have a DC component, proportional to the bias current, plus a varying component proportional to the signal current. Figure 5 presents the same argument graphically, and Fig. 6 shows a circuit

                

                             Fig. 5 Plot of collector current vs base current showing effect of bias and signal inputs. The dashed line indicates the bias current input and resulting collector current. Addition of a varying component causes the collector current to follow within the limits shown.

                    

                           

                                  Fig. 6. Biased NPN common-emitter amplifier. A capacitor is used to couple in the AC signal so that the signal source cannot affect the bias current. The same circuit works for a PNP transistor if all polarities are reversed.

implementation. Since the plot is not a straight line there will always be some distortion in the output signal, and signals large enough to drive the transistor into cut-off or saturation will be limited or 'clipped'. 

                                       The output voltage of the common emitter amplifier can be substantially larger than the input voltage. With no external signal, some amount of collector current flows, resulting in a voltage drop across the collector resistor which sets the output voltage between VCC and ground. A small positive signal input will cause additional base current to flow, which causes a much larger increase in the collector current, increasing the voltage drop across the collector resistor and lowering the output voltage. A small negative signal would decrease the base current, thereby increasing the output voltage. The output voltage is therefore an inverted and amplified replica of the input, offset by a DC component due to the bias current. The DC component can be blocked by another capacitor, leaving only the desired signal. A detailed analysis shows that the gain is determined by hFE and by the effective resistances at input and output. 

                                    The emitter resistor RE shown in Fig. 6 is not actually essential for circuit function, but it does serve a very useful purpose. The flow of collector current heats the junction because of inevitable losses. The current gain hFE increases at higher temperatures, resulting in still more collector current for the same bias current, further increasing the temperature. In the absence of an emitter resistor, this succession of events can destroy the device, a phenomenon known as thermal runaway. At best, it will shift the operating point toward saturation and may make the circuit inoperative. An emitter resistor reduces the tendency for thermal runaway by reducing the

                                

                                           

                                                 Fig. 7 The emitter-follower circuit.

voltage from base to emitter as the collector current increases. The decrease in VBE decreases the base current, partially compensating for the increased hFE and thereby stabilizing the circuit. This is an example of negative feedback, which we will see again later. Practically, an RE much smaller than RC is usually sufficient for stability. 

                        Sometimes one wants to transfer substantial power to a low resistance load. If the signal source cannot supply the power, the emitter follower circuit in Fig. 7 is useful. A small current from the signal source, applied to the base, can cause a large amount of current to flow through the emitter resistor or a load connected in parallel with it. Note that this amplifier does not invert: An increase in base voltage causes more collector current to flow, increasing the voltage at the emitter. It can be shown that the output voltage is only equal to the input voltage, minus the drop in the base-emitter diode, so there is no voltage gain. The circuit provides power gain, however, because there is a much larger current at the output than the input, and power is the product of current and voltage.

                            As drawn, the emitter follower can connect the load to the positive supply, but it cannot produce negative output voltages. Like the common emitter amplifier, the follower can be biased to amplify AC signals, but that leads to a steady current flow in the emitter resistor. If the circuit is intended for high power this implies large dissipation in the emitter resistor even in the absence of a signal. A much more interesting possibility is shown in Fig. 8. One transistor is NPN and the other PNP, connected between positive and negative supplies so that Q1 conducts on positive input swings and Q2 on negative inputs. With no input there is no collector current and hence no dissipation at all in the absence of a signal. The emitter resistor is shared by Q1 and Q2, and is usually the load to be driven rather than a separate circuit element. This circuit is called a push-pull emitter follower, or a complementary pair, and is widely used as the output stage of relatively high power amplifiers.

                                             
                                                        

Two failings of the push-pull follower are thermal runaway and crossover distortion. As in the common emitter amplifier, thermal runaway can be avoided by a small emitter resistor, typically an ohm or two, in each emitter lead. Crossover distortion occurs because the input signal must be large enough to turn on the base-emitter diode before the transistor will conduct. The circuit does not, therefore, produce an output until the input is bigger than about 0.6V, resulting in a distorted waveform. One cure is to bias the transistors so that they are on the verge.

                                              

                                        Fig. 9 A push-pull circuit with bias and runaway protection.

   

of conduction without a signal. This is shown in Fig. 9, where diodes are used to set the base voltage exactly one diode drop above or below zero. An alternative is to use feedback, as we will demonstrate later. The magnitude of voltage or power gain available from a single stage of amplification is obviously limited, so most practical amplifiers consist of several coupled stages. Further refinements are often added to improve characteristics such as frequency response or distortion for particular applications. Such complex devices are usually purchased, rather than being designed by an experimentalist, so they will not be considered here.

to be continued friends....in my next post..keep on reading and gaining knowledge....:)

 

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