In this module, students will learn:
TYPES OF AMPLIFIERS
In this section, I will discuss briefly the three different types of amplifiers one would encounter in a speech and hearing laboratory. Audiologists will probably find this section a bit more interesting than speech-language pathologists. SLPs who do any electromyographic work, or use voice instrumentation, should still find this of use.
Pre-amplifier. As the name suggests, a preamplifier comes before the "real" amplifier. Its purpose is to provide a relatively minor increase in voltage of the signal. A preamplifier may be built into equipment so that you don't even know that it is present. For example, a sound level meter or hearing aid microphone has a preamplifier. It provides enough power to boost the voltage of the signal well above the electrical "hum" inherent in electronic circuits. The improvement in the "signal-to-noise ratio" ensures that the signal will be measured / amplified cleanly. Without a pre-amp you would be amplifying a mix of the signal and electronic noise.
Amplifier, also called power amp. The amplifier provides the true boost in the power, and provides a signal large enough to "drive" the speaker. When hooking up equipment, the amplifier comes last, just before the transducer.
Differential amplifier or Op. Amp. (operational amplifier) or common mode rejection circuit. The purpose of this type of amp is to reduce "noise" present within a circuit and amplify only (OK, mostly, not only) the signal. A frequently encountered noise source is 60 cycle "hum" coming from the line (AC) power. Even with careful engineering, it is possible for some of the 60 cycle energy to become part of the electronic signal inside the instrument.
For an Op Amp to work, you need two "inputs". One of them contains the background noise, and one has the signal corrupted by the background noise. The differential amplifier only amplifies what is different between the two signals. While this sounds like high tech wizardry, it really isn't.
Here is an example of the two inputs - one is just noise (the top irregular sine wave) and one is noise plus a few cycles of a sine wave. We'll consider that noise plus signal.
Figure 1. Noise and noise plus signal waves illustrated.
The operational amplifier has two inputs, one market + and one marked -. The negative marked input is called the inverting input. The signal going into the inverting input is - inverted- turned upside down.
For technical correctness, a few points. When we are looking at these waves above, recall that the waves are representations of the electrical signal, which you know is showing how the electrons are flowing. For electron flow to take place, you need a positive and a negative wire. Below is a schematic of an operational amplifier - this is how it would look on an electronics diagram. If you were doing the wiring, you would take the positive wire of the signal (corrupted by noise) and put it to the connector representing the signal + input. The positive wire of the noise-only input goes to the amplifier's negative terminal. Both of the negative wires are connected together to the ground input. (If you don't have a specific wire representing signal plus noise, you take your ground and put it into the negative op amp input.)
Figure 2. Illustration of the schematic of an operational amplifier.
Inside the amplifier, the + input signal and the inverted signal coming from the "inverting input" are added together.
Adding these two waves together
Figure 3. Signal-plus-noise wave and the inverted noise-only wave.
Gives you the difference wave, which is this wave.
Figure 4. Result of the addition of the waves shown in figure 3.
A clinical application of op amps - electrophysiologic measures
When you are making a physiologic measurement - muscle potentials, auditory evoked responses, you are using differential amplifiers. So, if you never have the occasion to put a set of electrodes on someone, you're right, this isn't so exciting. For the rest of you... When you have to place three electrodes on someone in order to see even one "channel" of data, you are using an op amp. The + signal (e.g. the top of the head EP electrode) goes to the positive input. The signal called negative or inverting (e.g. earlobe) goes to the negative input. You then need a ground electrode from some neutral site, such as the opposite earlobe or the forehead. Both the inverting/ground and the non-inverting/ground electrodes pick up the general body current - containing huge voltages from the overall muscle and thought activity, as well as giving the basic body DC polarity. Common-mode rejection (differential amplification) cancels most of that out, so it becomes possible to see the difference in voltage between the two electrodes (the top of the head to the ipsilateral earlobe, for example.)
Audiologists who would like more information are referred to:
When two (or more) electric signals need to be combined, the proper thing to do is to use a mixer. The mixer contains several op-amps, so that circuit noise is reduced. Additionally, the mixer prevents distortion that can occur if you simply combined the two signals without a mixer.
CLASSIFICATIONS OF AMPLIFIERS
In the last section I talked about general types of amplifiers. This section concerns the different "classes" of amplifiers and their advantages, which will be of interest to dispensing audiologists, and those who like high fidelity sound systems.
This is the type of amplifier, powered by a battery, that has historically been used in hearing aids routinely. Hopefully its use will decrease. It can produce at most 1.3 volt peak-to-peak output, so it is not for use in power hearing aids. Above 1.3 volts the amplifier will clip. Audiologists have come to know this type circuit as the "crummy peak clipper".
Figure 5. Top. A hypothetical speech wave amplified to approximately 1.0 volts. The signal is amplified cleanly. Bottom. Illustration of the peak clipping that occurs at +/- 1.3 volts in a class A hearing aid amplifier. This peak clipping results in distortion.
Besides limiting loudness only by chopping off the tops of the sound wave (peak clipping), a class A amplifier has another disadvantage. It is an inefficient user of battery power. It will draw current even if it is in a quiet environment.
Class B amplifier, aka push-pull amplifier.
A class B amplifier is really two amps in one. One amplifier amplifies voltages that are positive, one amplifies the negative voltages. The quality of the circuit is influenced by how well the crossover between the negative and positive is handled.
Class B amps use less current because the amp that isn't working doesn't draw power, and if neither amp is active, no power is drawn. So, less battery drain if the user doesn't turn the hearing aid off over night.
Class D amplifiers.
Yes, there is a class C, but it is a radio frequency transmission amplifier, so we can skip right to class D. These were developed since the 1980s and use CMOS (complementary metal oxide semiconductor) technology. These high-efficiency amps have even class B beat. The power savings advantage depends upon the sound pressure level at the ear, but they can increase battery life by around 50 to 75%. For example, Starkey's custom ITE low-drain class D circuit with 107 dB SPL maximum output, 40 dB gain, 15 slope hearing aid has a 40-53 day (size 13) battery life. (Will an open zinc air cell even last that long?)
BATTERY LIFE CALCULATION
A section on batteries wouldnt be complete unless we review the calculation of battery life. Speech-language pathologists may not be interested and should feel free to skip this section. Many audiologists may already know this content, but for those who need a refresher . . . . The number of hours a battery will last depends upon both the battery and the hearing aid.
mAh ratings of battery cells
Zinc-air batteries (cells) capacity is measured in milliampere hours (mAh). A milliampere hour is how many hours a battery would last in the circuit required ("drew") 1 milliampere. A battery with 130 mAh would last 130 hours in a hearing aid that draws 1 mA.
Batteries of the same size differ slightly in their mAh ratings. For example, a 675 battery could have anywhere from 530 to 600 mAh. While some batteries list their mAh ratings, this is increasingly rare.
Batteries can also differ in whether they continue to drain when not in use. Zinc-air battery users know that this battery type certainly does drain when not in use, while the old mercury batteries didnt suffer from this limitation. (Of course the environmental harm and the health risk of mercury offset this limitation.)
How Hearing Aid Amplifier causes Differences in Battery Life
As discussed above, the amplifier design is one factor in the current requirement. Power, both in terms of gain and in maximum power output, also affects the amount of current used by an amplifier. Filtering will too. Active filtering (more to come on this point) requires power - so a hearing aid that amplifies only high frequencies may draw more current than one that amplifies a wide range of frequencies.
Perception of quality differences in amplifiers.
Hopefully, the improved battery life will make audiologists inclined to chose class D amplifiers. Audiologists tend to be eco-friendly and won't want to see the pollution of battery production increase needlessly. But in case there are those among us who think class D is bad because it will hurt their battery sales, let me plug class D for its quality improvement. As you will learn in the amplification course, class D amplifiers provide more "head room" - better ability to amplify high, transient voltages without distortion from peak clipping. This can provide better fidelity output.
I haven't scoured the literature exhaustively, but when D was coming out, there was a survey of hearing aid dispensers at a convention. They were asked to indicate which hearing aid had the best quality: those with A,B or D circuits. Here is a summary of the results of the golden ears of 110 dispensers.
|Behind the Ear Aid||In the Ear Aid|
Gosh, analysis of whether these differences are statistically significant would make a nice assignment for the statistics class!
But I digress. Back to the main point. Better quality = happier customers = more referrals.
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