Op amps make JFET circuits repeatable

Glen Brisebois, Linear Technology Corp, Milpitas, CA -- EDN, 5/24/2001

Because they use practically no bias current (a useful feature in itself), JFETs also have practically no current noise. This feature means that you can use JFETs in very-high-resistance circuits and obtain good noise performance. JFETs are also fast devices, with garden-variety types specified in the hundreds of megahertz. On the flip side, JFETs are difficult to exploit in a manufacturing environment because they have widely varying dc specifications. With a simple resistor-based bias network, the same part number can give results that differ by several volts from device to device. One way to use JFETs in a repeatable and manufacturable manner is to use the topology that Figure 1 shows. The purpose of the op amp is to bias the JFET at V_GS=0V and, therefore, at I_D=I_DSS. It meets this goal by increasing the current in the bipolar transistor until V_GS=0V and I_D=I_DSS.

In this condition, the JFET operates at its highest gain (g_m) and lowest voltage-noise condition. The JFET operates as a follower with zero offset. The only requirement for the op amp is that it have ultralow bias current. A variety of op-amp types satisfy this criterion, including JFET-input op amps, such as the LT1462; superbeta-input op amps, such as the LT1097; and micropower op amps, such as the LT1494. Figure 2 shows an implementation of the topology of Figure 1, using an inexpensive, fast 2N5486 JFET. This device specifies I_DSS at 8 to 20 mA at room temperature. The LT1097 maintains the gate-source voltage at 0V by adjusting the JFET's drain current. The source of the JFET connects to the inverting input of the 325-MHz, low-noise LT1806 op amp. R_F closes the loop back to the JFET's gate. In this application, the circuit serves as a transimpedance amplifier for a fast photodiode.

Selecting a high value, 10 MW for R₁ maintains low noise gain, but you could reasonably reduce it to a few times larger than R_F. The values of the other resistors and capacitors in the LT1097 loop to attenuate the noise and shape the noise bandwidth of the slow loop. Measurements show the output-noise spectral density is 9 nV/ with R_F=0W, so resistor noise dominates with R_F greater than approximately 10 kW. Table 1 shows the rise time and bandwidth achieved for several transimpedance gains (as set by R_F). To obtain optimum speed characteristics, you make 'parasitic-capacitance adjustments' (the capacitor with broken lines in Figure 2) by adjusting the proximity of R_F's leads to its body. Figure 3 shows the time-domain pulse response with R_F=1 MW. Connecting two 499-kW resistors in series improves the response.