This tutorial will concentrate on the “front end” of the modern audio amplifier design. In Addition to this, this tutorial establishes some of the basics relating to integrated circuit operational amplifiers.
Figure below is a kind of semi-block diagram showing the input stage of most high-power audio amplifiers. Q1, D1, D2, and P1 form a circuit called a constant-current source. For this discussion purposes, suppose that the D1 drops the same value of voltage as the base-emitter junction of Q does. That means that the voltage drop across D2 shall also be the voltage drop across P1. If the value of voltage drop across D2/P1 is 0.7 volt, and P1 is adjusted to be 700 ohms, the value of emitter current flow will be 1 milliamp. Because the collector current of Q1 will almost equal to the emitter current, the level of collector current is also “held” at 1 milliamp. It is important to Note here that the collector current is regulated; it does not depends on the collector load or the amplitude of the source voltage. The variables that are controlling the collector current are only D2’s forward threshold voltage and the setting of P1. Therefore, we can appropriately name it as a constant-current source. Transistors Q2 and Q3 form a differential amplifier.
It looks like a seesaw in a school playground. As long as everything is balanced on a seesaw, it keeps itself in a horizontal position. If something unbalances it, it tilts, causing one end to go up respectively, and the other end goes down. This is exactly the same as how a differential amplifier works with the current flow. As we talked previously, it is assumed that the constant current source is required to provide a regulated 1 milli-amp of current flow to the emitters of Q2 and Q3. If Q2 and Q3 are in a balanced condition, this 1 milliamp of current will divide equally between each transistor, providing 0.5 milliamp of current flow through each of the collector. If we apply input voltage between the two base inputs (A and B) so that point A is at a different potential than point B, the balance will become upset. But when collector current rises through one transistor, it must decrease by the equal value through the other, because the constant-current source will not allow a varying “total” current. Take this example, if the differential voltage between the inputs made the collector current through Q2 to rise to 0.6 milliamp, the collector current through Q3 will fall to 0.4 mille-amps. The total value of current through both transistors still adds up to 1 milliamp.
It looks like a seesaw in a school playground. As long as everything is balanced on a seesaw, it keeps itself in a horizontal position. If something unbalances it, it tilts, causing one end to go up respectively, and the other end goes down. This is exactly the same as how a differential amplifier works with the current flow. As we talked previously, it is assumed that the constant current source is required to provide a regulated 1 milli-amp of current flow to the emitters of Q2 and Q3. If Q2 and Q3 are in a balanced condition, this 1 milliamp of current will divide equally between each transistor, providing 0.5 milliamp of current flow through each of the collector. If we apply input voltage between the two base inputs (A and B) so that point A is at a different potential than point B, the balance will become upset. But when collector current rises through one transistor, it must decrease by the equal value through the other, because the constant-current source will not allow a varying “total” current. Take this example, if the differential voltage between the inputs made the collector current through Q2 to rise to 0.6 milliamp, the collector current through Q3 will fall to 0.4 mille-amps. The total value of current through both transistors still adds up to 1 milliamp.
Now we are going to talk about advantages of such a circuit.
Consider for a while that the source voltage (that is supplied externally) increases. In a common-transistor amplifier, an increase in the source voltage will lead to a corresponding change throughout the whole transistor circuit. In our Fig below, an increase in the source voltage (+V) does not make any increase in current flow coming from the constant current source, because it is controlled by the forward voltage drop across D2, which does not change with an increase in current. Q2 and Q3 would still possess a combined total current flow of 1 milliamp. The collector voltages of Q2 and Q3 will increase, and they will also increase by the same amounts, even if the circuit got in an unbalanced state.
As a result, the voltage differential between the 2 collectors will not change. For instance, imagine that Q2’s collector voltage is 6 volts and that of Q3’s collector voltage is 4 volts. If a voltmeter is used to measure the difference in voltage between the two collectors, it will measure 2 volts (6 - 4 = 2). Now suppose the source voltage increased by an amount that is sufficient to cause the collector voltages of Q2 and Q3 to increase by 1-volt. Q2’s collector voltage will go up to 7 volts, and Q3’s will rise up to 5-volts. This did not vary the voltage differential between these two collectors at all; it still remained at 2 volts. Or say in other words, the output of a differential amplifier is resistant to power supply fluctuations. It does not only apply to gradual changes in DC levels; but also the effect works well with power supply ripple and other sources of uninvited noise signals that may enter through the power supply.
The most common troubles with high-gain amplifiers is that of noise and interference signals being applied to the input through the input wires. Input wires and cables can pick up a variety of useless signals, just as an antenna is receptive to radio waves. If you have ever touched an un-insulated input to an amplifier, you un-doubt ably heard a loud 60-hertz roar (called “hum”) through the speaker. This is because our body picks up electro-magnetically radiated 60-hertz signals from power lines all around us. The Fluorescent lights are particularly bad electromagnetic radiators. Below is Figure shows an example of some electromagnetic radiation that cause noise pulses on the A and B inputs to the differential amplifier. As the electromagnetic radiation travels at the great speed of light (that is, 186,000 miles/second), the noise pulses will happen at the same time, and also in the same polarity. This effect is known common-mod interference. A very attractive attribute of differential amplifiers is that they exhibit common-mode rejection. The noise pulses demenstrated in our Figuer below will not be amplified.
0 comments:
Post a Comment