05 - Amplifiers Flashcards
Amplifiers are frequency-________ (dependent/independent)
Independent (all frequencies get scaled to the same degree)
True or False: Amplifiers require a power source
True (can’t get “something for nothing”)
Name two characteristics of a good amplifier
- Wide bandwidth (range of applicable frequencies)
- Low internal noise
- Low distortion
- Maximum efficiency (low loss of energy in amplification process)
Amplifiers, especially Fixed Gain Amplifiers, can cause distortion. One way to get around this is to tie the gain to the input, so as the input increases, the amplifier predicts the output, and uses a lower gain as needed (self-monitors). What are these amplifiers called?
Automatic Gain Control (AGC) Amplifiers
What is the name of the measure that quantifies the level of harmonics relative to the fundamental?
Total Harmonic Distortion (THD)
In an ideal amplifier, THD would = 0.
A good amplifier typically has <1% THD
What does IMD stand for and what does it quantify?
Intermodulation Distortion - it is measured using a combination of pure tones, and quantifies the relative levels of the intermodulation components
Name two types of amplifier (can be hearing aid specific or not)
Voltage amplifiers
Current amplifiers
Power amplifiers
Difference amplifiers
Hearing Aid Power Amplifiers: Class A amplifiers Class B amplifiers Class D amplifiers Class H amplifiers
What is the main disadvantage of Class A amplifiers?
Very inefficient - even when there is no microphone input signal, there is still current flowing through the loudspeaker at baseline resistance of the amplifier (quiescent current). They only transfer ~50% of the battery power to the hearing aid receiver, but if we tried dropping the quiescent current, we would get clipping below 0.
Class B Amplifiers are known as __________ Amplifiers, and get around the problems of Class ___ Amplifiers
Known as “Push-pull” amplifiers
Get around the problems of Class A amplifiers by using two separate amplifiers, each with zero or very small quiescent current, and an inverter. Each amplifier handles one half of the waveform (positive or negative half).
Class B amplifiers have better efficiency than Class A amplifiers - about 79%
Class B amplifiers are particularly suitable for high-power amplifiers
Which technique do Class D amplifiers use to drive the receiver?
Pulse Width Modulation (PWM) - the amplifier generates high frequency pulses of varying width in response to input signal
- longer pulse widths represent higher amplitudes
- receiver smoothes out the pulses and produces the amplified signal
- theoretical power efficiency = 100%; practical efficiency = ~90%
Which Class of amplifiers is the most popular for hearing aids?
Class D
With Class D Amplifiers, the voice coil is physically incapable of moving at such a high rate, so what needs to happen for the voice coil to get “pushed”?
There needs to be more pulses and more duration at the positive side to push the voice coil (the same is true at the negative side to pull the voice coil).
If the pulses and duration are equal in the positive and negative domains, the voice coil doesn’t have time to respond, which results in a value of 0 (and leads to no quiescent current)
Explain how we use differential amplification to measure ABR (Acoustic Brainstem Response)
- ABR takes in 2 inputs (one non-inverting, and one inverting) and the difference between the two is what is actually amplified (differential amplifier)
- the positive lead has the signal of interest + background noise, whereas the negative lead just has the background noise, so this signal can be used to attenuate the common mode components, leaving the signal of interest
- (still need to do signal averages on signal of interest)
What does CMRR stand for?
Common Mode Rejection Ratio - it specifies the performance of the differential amplifier
When will distortion occur in an amplifier?
When the input signal is too big, or when the power supply to the amplifier is inadequate
