| Model: | - |
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| Brand: | - |
| Origin: | Made In United States |
| Category: | Electronics & Electricity / Electronic Components / Other Electronic Components |
| Label: | LSK189 |
| Price: |
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| Min. Order: | - |
| Last Online:06 Dec, 2024 |
| Specifications: | LSK189 TO-92 3L LSK189 SOT-23 3L |
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| Advantages: | A low noise JFET such as an LSK189 is configured as a source follower, with a source biasing resistor R3. In terms of input capacitance at the gate of J1 with an LSK189, it is about 3 pF from the gate to the drain, which is much less than the 18 pF of Cin(-). Capacitance between the gate and ground due to the gate to source capacitance approaches zero. This is because the source follower configuration provides substantially the same AC voltage at the gate and at the source, which substantially cancels out the capacitance between the gate and the source. The source follower also greatly reduces the capacitance seen at the gate to ground even when the source is driving signal into a capacitive load, Cin(-), the capacitance at the (-) input terminal of the op amp U1A. It should be noted that the source follower circuit may add some phase shift to the overall amplifier circuit. To ensure phase margin and no oscillations, a resistive divider R1 and R2 is used. With the values given at 3900Ω for R1 and R2, the equivalent feedback resistance is [1 + (R2/R1)] x RF = 2 x 1MΩ = 2MΩ, the same resistance value shown in Figure 8 for RF. If the (+) input of the op amp in Figure 9 is grounded, such that Voffset = 0 volts, Vout will most likely have a DC offset. To “zero” Vout when there is no signal from the photodiode, a clean DC voltage, Voffset may be applied to the (+) input of U1. Op amp U1A = AD797 has an equivalent input noise voltage of 0.9 nV/ �� and J1 =LSK189 with an equivalent input noise voltage of 1.8 nV/ ��, the total equivalent input noise voltage is about 2.0 nV/ ��. This is lower than a very low noise JFET op amp such as an AD743 that has 3.2 nV/ ��. Note that bipolar input stage op amps AD797 with 0.9 nV/ �� has lower equivalent input noise voltage than 2.0 nV/ ��, but the AD797’s input noise current is too high and are not suitable for amplifier circuits with large value feedback resistors (RF) in the MΩ such as the circuit shown in Figure 8. Note that J1 may be substituted with an LSK170 (0.9nV/ �� ) if a slight increase in capacitance from the gate to ground is acceptable. This FET has about half the equivalent input noise of the LSK189. |
