A few days ago we opened the electronics workshop with some theory and practice of multiplexers and demultiplexers , this week we will explain what an amplifier , its characteristics and what to keep in mind to differentiate between them . Take note!
In short: an amplifier is a device capable of magnifying what passes through it .
The relationship between the amplifier output and the input, normally expressed in terms of the frequency of the input signal, called transfer function of the amplifier gain and amplitude . Depending on the amplification of the frequency, they are made to operate at a given frequency range so that the amplification is linear and constant.
The electronic amplifiers are the most common and are used in almost all electronic devices such as radios, televisions, computers, communications equipment, musical instruments, and so on .
The operation of these amplifiers are based on the use of different active elements , such as vacuum tubes or transistor normally BJT, MOSFET although sometimes used. BJT function is to amplify the electrical current has at its base a certain value in the collector and emitter. The amplification value depends on the type of transistor and circuit design (component values, in common base configuration, common rail, etc.).
Transistors can be made more complex devices also fulfill the function of amplifying, such as operational amplifiers , and these in turn others such as instrumentation amplifiers .
Another type of electronic amplifiers are designed specifically for audio , they are often preferred in vacuum tubes to transistors for best sound characteristics. These audio amplifiers are the preamplifiers and power amplifiers .
A audio amplifier it is also known as power amplifier, power amplifier or gain stage.
The main purpose of the amplifier is increasing the level of input signal , for that purpose must be increased amplitude of the input signal by bias current in the output transistor.
When an amplifier is designed it is essential to vent it. Therefore, we will always find vent and manufacturers have installed inside fans (and your computer) or a heat sink. This is because during the signal processing large amount of heat is released in its interior.
Physically, when we see an amplifier, we find a team where usually there is only one button: the power to plug or off.
In the back, however, it is the panel with corresponding inputs and outputs of which will depend on the amount of signals that can withstand a given amplifier model.
These characteristics are what determine the quality of each amp model:
- Damping factor.
- Output power.
- Frequency response.
- Phase response.
- Impedance is the opposition presented any material or device to the passage of an alternating current. The input impedance of an amplifier should be at least 10 kilohms . These 10 kilohms exist for positioning in case of parallel amplifiers 10 overall charge of a kiloohmio (10 kiloohmio / 10 = 1 kiloohmio).
- Damping factor: indicates the relationship between the output impedance of the amplifier and the nominal impedance of the loudspeaker which is connected. The higher the damping factor better, but over two hundred may mean that the amplifier is poorly protected against reactive loads that can damage it .
Ideally, the damping factor is high, above two hundred.
The damping factor is expressed: ‘200 into 8 ohms’, meaning that the damping factor is 0.04 ohms (8/200).
Many manufacturers include the damping factor for serious, which is very useful, because we know that’s the critical frequency response. It would be indicated as ‘150 into 8 ohms at 40Hz’.
- Output: is the amount of energy that can be introduced into the power stage before distorting excessively or may suffer any damage.
The maximum power of the amplifier is a function specified impedance generally 8 ohms. For example: 175 watts into 8 ohms.
In amplifier is stereo, you have to consider whether that power relates to each of the channels or both. These must be specified in the technical specifications, if power is fed to the two channels, or channel.
In the above example with an output power of 175 watts into 8 ohms, if added ‘with the two channels fed’ means ‘channel’ power will be half (87.5 watts into 8 ohms).
By contrast, with an output power of 175 watts into 8 ohms ‘channel’, we 350W into 8 ohms ‘fed with the two channels’.
On computers that allow you to modify the input impedance, we must also take into account changes that vary the parameters introduced into power. In this case, closest approximations are made, they are never absolute, because in the current state of the amplifiers this is not possible. Thus, we have an amplifier in which the technical specifications contained 175 watts into 8 ohms, if we reduce the impedance 4 ohms, the power will be nearly double the 350 watt (is an ideal amplifier should be just these 350 watts) .
Within the power difference between nominal power and peak power.
Nominal effective or RMS power: is the maximum power that can withstand the amplifier in continuous regime, before deteriorating. If the amplifier works over this nominal power is irretrievably damage both the speaker to which it is connected as the amplifier.
- RMS , it is the acronym for Men Square Root, or what is the same rms value. Peak power, permissible or musical.
Impulsive maximum power (a peak signal) which can support time to time the amplifier before deteriorating.
Some manufacturers instead of specifying the rated power, specify the peak power to mask the scope of the amplifier, since the peak power is always higher than the rated power. We must be alert to this detail and take into account that the peak power of an amplifier is 0.707 times its peak value.
- Signal / noise ratio: refers to the residual voltage output noise and is expressed in dB. For the signal / noise ratio is below hearing threshold, it must be at least 100 dB. Mayor, 110 dB, in the case of high power amplifiers (above 200 watts).
- Coupling : indicates how the amplifier is connected to the speaker. There is talk of direct coupling when both are coupled directly, without intermediate steps.
Internally, the amplifier operates with voltage, but the output signal converted into alternating current. When directly connect an amplifier with the loudspeaker, this direct coupling must be made so that the residual current (DC offsets) is as low as possible, not exceeding 40 millivolts. The most common are at 15 millivolts.
- Frequency response: calculates the limit within which the amplifier responds equally (flat response) to the audio frequencies (20 to 20,000 Hz) with a very low power.
This frequency response is measured in dB with reference to the 1 Watt with impedance of 8 ohms. Frequents optimal response is about 5 dB above +5 dB or below -5 dB.
Many manufacturers, instead of using only the audio frequencies, to protect the amplifiers suprasónicas or subsonic disturbances, which make is to measure the frequency response for a higher frequency band (usually 12 to 40,000Hz). In this case a response in optimum frequency should be around 3 dB above +3 dB or -3 dB down.
- Phase Response: related phase between middle frequencies relative to the high or low. This phase shift (or delay overtaking) should not exceed 15 ° for not distortion and / or CANCELLATION POLICY signal occurs.
There are some amp models that reverses the phase throughout its passband, which may cause difficulties in their operation (but what we remember we be canceling all signal).
- Gain is the ratio between the output power and the input power signal. If the output power is 40W (watts) and the input of 20W, the gain is 2 dB.
When the gain is less than 1, we talk attenuation. With regard to amplifiers, as always express decibel a comparison DBW talk or dBu, which will indicate which is the reference:
- dBw : The W indicates that the decibel refers to watts. That is to say, is taken as reference 1 W. Thus, one watt is equal to 0 dBW.
- dBm : when the value in Watts is very high, it is one milliwatt (mW). Thus, a 0 dBm mW correspond.
- dBu : the signal level expressed in decibels, referred to 0.775 volts. The 0,775 are the voltage applied to a 600 Ohm impedance, it develops a power of 1mW. 600 ohm impedance is chosen, because this is where the signal level in dBm and coincides dBu.
When a circuit involving several amplifiers, individual earnings expressed in decibels in any of its forms (both dB, dBW dBm or dBu) add (subtract if they are negative and attenuation).
- Sensitivity indicates the amount of necessary electrical input stream to produce maximum output power.
DBu is indicated by a certain impedance. The dBu expresses the signal level of decibels, referred to 0.775 volts.
If the value specified by the sensitivity is exceeded the output signal will suffer a cut (both above and below), as in the limiters, and will be distorted so that can damage some equipment and the tweeter.
To avoid this major problem, most professional teams has a control input level, allowing us to attenuate the signal if it is excessive.
- Distortion describes the variation of the waveform at the output, and with respect to the signal entered and because audio equipment, not only amplifiers, harmonics introduced in the signal. The causes of this distortion can be multiple. In the case of amplifiers, the most common is the input overload, ie exceed the manufacturer ‘s recommended power, producing outputs a signal clipping, is broken sound.
The total harmonic distortion be at most 0.1% THD (total harmonic distortion), across the spectrum (high frequency – acute, distort more than low – severe).
Distortion can also be expressed in dB relative to frequency. It is what is known as intermodulation distortion transistors. To measure this distortion what is done is to calculate the distortion amplifier for two different sine waves (generally, 19 and 20 kHz) and see what the difference between these signals expressed in dB. Quality amplifiers must be 70 dB difference in that tone differential of 1 hhz.
- Crosstalk : Crosstalk indicates that a stereo system, an audio channel affects the other. Crosstalk depends on the frequency. So we ‘ll talk about that crosstalk is bearable when around 50 dB for bass and treble and 70 dB for halftones. To remove crosstalk problems, the amplifiers have rectifiers, filter capacitors. In addition, many manufacturers use independent sources of power for each channel, which is very effective.
If you want to practice this lesson, you can do with this assembly .
To learn more electronics, check out this course , you may be interested!
We hope this comprehensive lesson amplifiers will be useful for your projects, if you want another items you can explain the types that exist. Do you want? We look forward to your comments or suggestions on any subject!