1. Eulogio Amang Rodriguez Institute of Science and Technology Nagtahan, Sampaloc, Manila Vacuum Tubes Amplifiers

2. Introduction The process of raising the strength of a weak signal is known as amplification. The device which accomplishes this job is called an amplifier. All grid controlled Tubes (e.g. triode, tetrode, pentode etc.) can act as amplifiers. The weak signal is applied between control grid and cathode and the amplified output is obtained across the load in the plate circuit.

3. The amplifying property of a grid voltage causes a relatively large change in plate current and hence greater output is Obtained across the load in the plate circuit i.e. grid voltage is amplified. It should be noted that an amplifier amplifies only the electrical signal. This signal to be amplified may be directly available or converted into electrical By some suitable device.

4. Triode as an Amplifier A triode is a grid controlled tube and can thus act as an amplifier. The weak signal is introduced in the grid circuit and amplified output is obtained across the load in the plate circuit. In order to achieve proper amplification from a triode, it is necessary that The control grid remains negative w.r.t. cathode at all times.

5. If grid is driven positive at any time by the input signal, the grid will draw current, Resulting in power loss and distorted amplification. To ensure that grid is always negative w.r.t. cathode, one method can to connect a battery E c in the grid circuit in addition to the signal. This d.c. voltage is known as grid bias and is always of sufficient Magnitude and of such polarity so as to maintain grid negative w.r.t. cathode at all times. Batteries are costly and require frequent replacement and therefore, they are Never used to provide gird bias in vacuum tube amplifiers.

6. In practice, d.c. component Of plate current of triode itself is used for obtaining grid bias voltage. The triode is then said to be self-biased. Two commonly used methods of self- biasing a triode are: cathode bias and grid leak bias. Consider a triode amplifier having a.c. resistance of r p, load resistance R L and amplification factor μ. then it can shown that voltage gain Av of the circuit is given by; Av = μ R L R p + R L NOTE: It may be seen that the voltage gain of the circuit is less than u. it is because some voltage is dropped across the internal resistance r p of the tube. If r p = 0, then Av = μ.

7. Pentode as an Amplifier As pentode is also a grid controlled tube, therefore, it can act as an amplifier like a triode. However, the amplification obtained in a pentode circuit is far superior to that realised in a triode circuit. This is expected because a pentode has very high value of u and little plate to Grid feedback. The voltage gain of a pentode amplifier can be calculated in a manner similar To a triode amplifier and is given by ; Av = μ R L r p + R L

8. Where u, r p, and RL are the amplification factor, a.c. plate resistance and load resistance respectively of the pentode circuit. Generally r p >> R L so that the latter can be neglected As compared to the former. With this assumption, the voltage gain of a pentode amplifier becomes: Av = μ R L r p = 8 R L ( g m = μ / r p ) NOTE: This expression for voltage gain of a pentode amplifier is fairly accurate and can be applied In practical circuits.

9. Multistage Amplifiers The output from a single stage amplifier is usually insufficient to drive an output Device, such as a loudspeaker. Additional amplification over three to six stages is generally necessary. To accomplish this, output of one stage is electrically coupled to the next stage through a suitable coupling device. The resulting system is referred to as multistage Amplifier. The most commonly used methods of coupling are: (i) R-C coupling (ii) Choke-capacitance coupling (iii) Transformer coupling

10. In each method of coupling, the object is to transfer a.c. output of one stage to the input of the next stage and to isolate the d.c. conditions of one stage from the other. It may be noted that the use of particular coupling depends upon service requirements. For example, R-C coupling is best suited for voltage amplification because of its distortionless amplification. On the other hand, transformer coupling is invariably Employed for power amplification because it permits impedance matching.

11. Power Amplifiers An amplifier which raises the power level of the signal is called amplifier. The last stage of a multistage amplifier is usually the power stage. Here, concentrated effort is made to transfer maximum power to the load. Obviously, transformer coupling would be en appropriate device for such a situation. Power amplifiers are generally classified on their mode one operation i.e. plate current flow during various parts of the signal.

12. i) A class A power amplifier is one in which grid bias is so adjusted that plate current flows during all parts of the signal. Class A operation gives distortion less output but has the major limitation that plate efficiency is low (≡30%). (ii) A class B power amplifier is one in which grid bias is so adjusted that plate current flows during the positive half cycle of the signal only. Class B operation gives distorted output but has the advantage that plate efficiency is quiet high (≡50%). Most of the power amplifiers employ two tubes in class B operation; the circuit being known as push-pull circuit. One tube amplifiers the positive half-cycle of the signal.

13. (iii) A class C power amplifier is one in which grid bias is so adjusted that plate current flows for the less than half-cycle of the input a.c. signal. The features of class C operation are: higher distortion, high power output and excellent plate efficiency (≡75%). Because of their inherent distortion, the use of class C amplifiers is only limited to oscillators and final stage of radio transmitters where higher efficiency rather than distortion is important.

14. Power Amplifiers An amplifier which raises the power level of the signal is called amplifier. The last stage of a multistage amplifier is usually the power stage. Here, concentrated effort is made to transfer maximum power to the load. Obviously, transformer coupling would be en appropriate device for such a situation. Power amplifiers are generally classified on their mode one operation i.e. plate Current flow during various parts of the signal.

15. (i) A class A power amplifier is one in which grid bias is so adjusted that plate current flows during all parts of the signal. Class A operation gives distortion less output but has the major limitation that plate efficiency is low (≡30%). (ii) A class B power amplifier is one in which grid bias is so adjusted that plate current flows during the positive half cycle of the signal only. Class B operation gives Distorted output but has the advantage that plate efficiency is quiet high (≡50%).Most of the power amplifiers employ two tubes in class B operation; the circuit being known as push-pull circuit. One tube amplifiers the positive half-cycle of the signal.

16. Because of their inherent distortion, the use of class C amplifiers is only limited to oscillators and final stage of radio transmitters where higher efficiency rather than distortion is important.

18. The following terms are often used in the analysis of power amplifiers: (i)Plate power input(P in ).This refers to the power supplied to the plate circuit by the d.c. power supply and is equal to the product of the d.c. plate potential and plate current i.e., P in =E b I b Important Terms in Power Amplifiers

19. (ii) Plate power output (P o ).This refers to the a.c. output from the tube and is equal to the product of the a.c. component of plate voltage and plate current i.e. P o =E p I p

20. (iii) Plate dissipation (P dis ). This refers to the power lost inside the tube in the form of heat as a result of electron bombardment of the plate structure. P dis =P in -P o (iv) Plate efficiency. This is the ratio of a.c. output power and d.c. input power i.e. Plate ῃ=(P o /P in )x100%