Chapter 14: Amplifiers & Oscillators

1. Chapter 14: Amplifiers & Oscillators

2. Amplifiers: Overview • Circuits which increase: voltage or current • Take small input signal to reproduce output waveform as larger amplitude • Ie. circuits which provide gain • Frequency selective (like a band-pass filter) • Audio frequency • Video (Cable / Satellite) • Radio

3. Design Criteria: Bias & Gain • When designing or selecting an amplifier, there are a couple of things to consider: • Bias • Recall three common types of bias: • Class A, B, C • Less common ones: Class AB1, AB2 • Gain • The amount of output power compared to the input power

4. Bias: Class A • Amplifies 360° of the waveform • Provides the best linearity (least distortion) • Less than 50% efficient • Commonly referred to as a “linear amplifier” • Generally the “PA (power amplifier) stage” of a transmitter is a class-A circuit

5. Bias: Class B • Amplifies only 180° of the waveform • Allows greater gain, but at the expense of linearity • How do you deal with the missing portion of the waveform? • Push-pull circuits • Two amplifiers operate in tandem, but 180° out of phase, amplifying the whole signal • Flywheel effect • The “missing” 180° is reproduced by the circulating current in an accompanying tuned circuit (recall the properties of a parallel-LC cct.) • 60-65% efficient

6. Bias: Class C • Amplifies only 120° of the waveform • provides the greatest gain but also the least linear bias • Excellent for non-AM modes • Can not be used with AM due to extreme non-linearity (distortion of waves = distortion of intelligence) • ~75% efficient

7. Gain: Powerout / Powerin • Gain is the ratio between the input signal power and the output of the amplifier Power gain: Gain (dB) = 10 log (Pout / Pin) Voltage gain: Gain (dB) = 20 log (Eout / Ein) • this works for both Voltage and Current!

8. Decibel Math: Review Q: If an amplifier provides 36db(!!) gain, what would the output power of an amplifier be if you were to “drive” it with 1W of power? • Recall: • 3dB = 2x • 10dB = 10x • therefore count in groupings of 3db or 10db and multiply each of the “grouping” values • 36dB = • 10 * 10 * 10 • Answer: 4000W ! plus 2 groups of 3dB 3 groups of 10dB * 2 * 2

9. Linking Circuits: Coupling • We often use simple wire to connect circuits, however, sometimes it is necessary to isolate AC circuits from DC influences • It is possible to provide coupling of AC signals while isolating DC currents • Two common methods: • Transformers • Capacitors

10. Filters: General Function • Designed to couple circuits while excluding unwanted energy from the circuit • Pass or reject energy based on frequency • Common types of filters are: • Low pass • High Pass • Band Pass • Band Stop • Used extensively through out electronics

11. Filters: Low-Pass • Allows low frequencies to pass with minimal attenuation while blocking all frequencies above the cut-off frequency • Often used between TXVR and antenna f = low f = high Low frequency passed on to RL High frequency diverted thru C

12. Filters: High-Pass • Allows high frequencies to pass with minimal attenuation while blocking all frequencies below the cut-off frequency • Used to be common as TVI filters

13. Filters: Band-Pass • Allows a range of frequencies to pass, rejecting those above and below the cut-off frequencies

14. Filters: Band-Stop • Block a range of frequencies • In radio terms, often knows as a “notch filter”

15. Filters: π • Many low-pass filters use what is known as a Pi-filter configuration • As you increase the number of π-elements, you improve the filter’s selectivity (performance)

16. Decoupling • Decoupling is used where components are sensitive to change in the input signal • Often called a buffering stage • Provides the ability to isolate circuits • Prevents DC current on AC signal line • Most common methods • Capacitors • Transformers

17. Increasing Gain: Cascading • Vacuum tubes and transistors can only provide so much gain • Vacuum tubes provide voltage gain • Transistors provide current gain • To increase total gain of an amplifier, it is often necessary to have multiple amplification stages linked in series • This is known as cascading

18. Feedback: Can be useful