TYPICAL RF AMPLIFIER CIRCUITS.
As a technician, you will see many different rf amplifiers in many different pieces of equipment. The particular circuit configuration used for an rf amplifier will depend upon how that amplifier is used. In the final part of this chapter, you will be shown some typical rf amplifier circuits.
Figure 2-19 is the schematic diagram of a typical rf amplifier that is used in an AM radio receiver. In figure 2-19, the input circuit is the antenna of the radio (L1-a coil) which forms part of an LC circuit which is tuned to the desired station by variable capacitor C1. L1 is wound on the same core as L2, which couples the input signal through C2 to the transistor (Q1). R1 is used to provide proper bias to Q1 from the base power supply (VBB). R2 provides proper bias to the emitter of Q1, and C3 is used to bypass R2. The primary of T1 and capacitor C4 form a parallel LC circuit which acts as the load for Q1. This LC circuit is tuned by C4, which is ganged to C1 allowing the antenna and the LC circuit to be tuned together. The primary of T1 is center-tapped to provide proper impedance matching with Q1.
Figure 2-19. - Typical AM radio rf amplifier.
You may notice that no neutralization is shown in this circuit. This circuit is designed for the AM broadcast band (535 kHz - 1605 kHz).
At these relatively low rf frequencies the degenerative feedback caused by base-to-collector interelectrode capacitance is minor and, therefore, the amplifier does not need neutralization.
Figure 2-20 is a typical rf amplifier used in a vhf television receiver. The input-signal-developing circuit for this amplifier is made up of L1, C1, and C2. The inductor tunes the input-signal-developing circuit for the proper TV channel. (L1 can be switched out of the circuit and another inductor switched in to the circuit by the channel selector.) R1 provides proper bias to Q1 from the base supply voltage (VBB). Q1 is the transistor. Notice that the case of Q1 (the dotted circle around the transistor symbol) is shown to be grounded. The case must be grounded because of the high frequencies (54 MHz - 217 MHz) used by the circuit. R2 provides proper bias from the emitter of Q1, and C3 is used to bypass R2. C5 and L2 are a parallel LC circuit which acts as the load for Q1. The LC circuit is tuned by L2 which is switched in to and out of the LC circuit by the channel selector. L3 and C6 are a parallel LC circuit which develops the signal for the next stage. The parallel LC circuit is tuned by L3 which is switched in to and out of the LC circuit by the channel selector along with L1 and L2. (L1, L2, and L3 are actually part of a bank of inductors. L1, L2, and L3 are in the circuit when the channel selector is on channel 2. For other channels, another group of three inductors would be used in the circuit.) R3 develops a signal which is fed through C4 to provide neutralization. This counteracts the effects of the interelectrode capacitance from the base to the collector of Q1. C7 is used to isolate the rf signal from the collector power supply (VCC).
Figure 2-20. - Typical vhf television rf amplifier.
Q.35 What components form the input-signal-developing impedance for the amplifier?
Q.36 What is the purpose of R1?
Q.37 What is the purpose of R2?
Q.38 If C4 were removed from the circuit, what would happen to the output of the amplifier?
Q.39 What components form the load for Q1?
Q.40 How many tuned parallel LC circuits are shown in this schematic?
Q.41 What do the dotted lines connecting C1, C2, C5, and C6 indicate?
Q.42 What is the purpose of C3?
SUMMARY OF VIDEO AND RF AMPLIFIERS
This chapter has presented information on video and rf amplifiers. The information that follows summarizes the important points of this chapter.
A FREQUENCY-RESPONSE CURVE will enable you to determine the BANDWIDTH and the UPPER and LOWER FREQUENCY LIMITS of an amplifier.
VIDEO AMPLIFIERS must have a Frequency response of 10 hertz to 6 megahertz (10 Hz - 6 MHz). To provide this Frequency response, both high- and low-frequency compensation must be used.
PEAKING COILS are used in video amplifiers to overcome the high-frequency limitations caused by the capacitance of the circuit.
SERIES PEAKING is accomplished by a peaking coil in series with the output-signal path.
COMBINATION PEAKING is accomplished by using both series and shunt peaking.
TRANSFORMER COUPLING is the most common form of coupling in rf amplifiers. This coupling is accomplished by the use of rf transformers as part of the frequency-determining network for the amplifier.
ADEQUATE BANDPASS is accomplished by optimum coupling in the rf transformer or by the use of a SWAMPING RESISTOR.
NEUTRALIZATION in an rf amplifier provides feedback (usually positive) to overcome the effects caused by the base-to-collector interelectrode capacitance.
ANSWERS TO QUESTIONS Q1. THROUGH Q42.
A1. The difference between the upper and lower frequency limits of an amplifier.
A2. The half-power points of a frequency-response curve. The upper and lower limits of the band of frequencies for which the amplifier is most effective.
A3. (A) f2 = 80 kHz, f1 = 30 kHz, BW = 50 kHz(B) f2 = 4 kHz, f1 = 2 kHz, BW = 2 kHz
A4. The capacitance and inductance of the circuit and the interelectrode capacitance of the transistor.
A5. Negative (degenerative) feedback.
A6. It decreases.
A7. It increases.
A8. The capacitance of the circuit.
A9. Peaking coils.
A10. The relationship of the components to the output-signal path.
A11. Combination peaking.
A12. The coupling capacitor (C3).
A13. A shunt peaking coil for Q2.
A14. A decoupling capacitor for the effects of R2.
A15. A part of the low-frequency compensation network for Q1.
A16. A series peaking coil for Q1.
A17. A swamping resistor for L2.
A18. L1, L2, and R5.
A19. R9 and C5.
A20. The gain increases.
A21. The gain decreases.
A22. To provide maximum impedance at the desired frequency.
A24. By changing the value.
A25. Transformer coupling.
A26. It uses fewer components than capacitive coupling and can provide an increase in gain.
A27. A step-down transformer.
A28. A too-narrow bandpass.
A29. By using an optimumly-coupled transformer.
A30. Low gain at the center frequency.
A31. A swamping resistor in parallel with the tuned circuit.
A32. RF transformers are used and the transistor is neutralized.
A33. Degenerative or negative.
A34. By neutralization such as the use of a capacitor to provide regenerative (positive) feedback.
A35. C2 and the secondary of T1.
A36. R1 provides the proper bias to the base of Q1 from V BB.
A37. R2 provides the proper bias to the emitter of Q1.
A38. The output would decrease. (C4 decouples R2 preventing degenerative feedback from R2.)
A39. C5 and the primary of T2.
A41. The dotted lines indicate that these capacitors are "ganged" and are tuned together with a single control.
A42. C3 provides neutralization for Q1.