This style of amplifier is called a common-source and generally has a gain greater than one-that is, the change in output voltage is greater than the change in input voltage-true amplification. You may also see some preamp designs on the web where there is also a resistor connected to the drain of the JFET. 10k ohm is a good middle-of-the-road value. If the value of the resistor is too high, it will heat up (slightly) as current passes through it, which can result in noise. I've seen designs on the web where the size of this resistor ranges from 1k ohm to 220k ohm. If the value of this resistor is high enough, the gain will be very close to 1 and if the resistor is smaller, than the gain will be somewhat less than 1. The value of the source resistor affects the gain of the amplifier slightly. This configuration of an amplifier, where there is a resistor connected to the source of the JFET and the output voltage "follows" the input voltage, is called a source-follower. The ratio of the change in Vout over the change in Vin (the slope of the line) is the gain of the amplifier, which in this case is just equal to 1. In fact, for every increase of 1V of Vin, Vout also increases by 1V. In the range between Vin = -1.25V and Vin = 7.5V, Vout increases linearly with Vin. Vout goes to 0V when Vin is around -1.25V, which is the point where the JFET turns off. Note that when Vin = 0V, Vout = 1V, indicating that current is flowing through R1. The plot below shows the relationship between the input voltage Vin and the output voltage Vout, given a supply voltage Vdd = 9V and a source resistor of 10k ohms. One of the unusual things about a JFET is that if the input voltage is 0, meaning that the input is grounded, there is still a current that will flow from drain to source, and you actually need to apply a negative voltage to the gate to turn the device off. Having nowhere else to go, the current flows through the source resistor R1, which in turn produces a voltage across R1 (remember Ohm's Law, V = IR), causing the output voltage Vout to also increase. In general, increasing the voltage Vin on the gate of the JFET causes an increase in current flowing through the device from the drain to the source. The vast majority of contact mic preamp designs found on the web also use JFETs.Ī Simpler Example: The Basic JFET Source-Follower AmplifierĪs a starting point to understanding the complete preamp, let's consider a simpler version shown below:Ī JFET has 3 terminals called the gate (g), drain (d), and source (s). JFETs have a very high input impedance and are less fragile and noisy than MOSFET, so that's what we'll use. MOSFETs have an extremely high input impedance, but tend to be fragile (static electricity can destroy them) and also can be noisy in audio applications. Bipolar transistors don't have a high enough input impedance. There are three types of transistors to consider, bipolar (BJT), MOSFET, and JFET. It also provides more current to drive signals on the cable to the amp, which can also improve the highs.Īmplifiers use transistors (or tubes) to amplify or buffer a signal. The role of the piezo contact mic preamp isn't really to amplify the signal (the output voltage of the piezo is plenty high), rather the goal is to provide a buffer with a very high input impedance of 10M ohms or so, which is more than 10 times or so greater than the input impedance of a typical guitar amp, so that more of the bass gets through. In order to reduce the cutoff frequency so that more bass tones get through, we need to increase the input impedance that the piezo is driving. For typical values of Cpiezo and Ramp, the cutoff frequency is well into the audible playing range of a guitar. Together, Cpiezo and Ramp form a high-pass filter that cuts out low frequencies. The input of an amplifier acts like a resistor to ground (Ramp), known as its input impedance. Because of the series capacitance (Cpiezo), virtually no DC current flows through the piezo. This voltage may be high, but the current is very small. To understand how it works, we'll first take a look at the problem we're trying to solve, and then build up a solution in steps.Ī piezo element operates like a voltage source in series with a capacitor-when the piezo crystals vibrate, they produce a voltage. The schematic above is the complete design of the preamp circuit.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |