2 Theory of Operation

The JFET preamplifier (preamp) circuit is easiest to analyze using the small-signal T-model as shown in #GUID-54BF9E97-2026-4543-958B-9001AB9130E2. To understand the operation of this circuit, begin by examining the preamp at the input. A sensor generates a small-signal input voltage (v_in), which modulates the gate-to-source voltage (v_gs) of the JFET. The JFE2140 is the first gain stage in the preamp circuit and conducts a small-signal drain-to-source current i_ds1 = g_m1 × v_gs1 that fluctuates with v_in. The transconductance gain parameter (g_m), is expressed in Siemens and v_gs is expressed in volts.

The small-signal current i_ds1 flows through resistors R_D1 and R_D2 forming a differential voltage v_od between the drains of the JFE2140. The OPA202 monitors v_od and drives the loop with voltage v_out such that the inputs of the OPA202 are approximately equal. The JFE2140 combined with the OPA202 form the feedforward gain stage A shown in #GUID-54BF9E97-2026-4543-958B-9001AB9130E2. Assuming symmetry and i_ds1 = i_ds2, the gain of the JFE2140 is g_m × R_D. The transconductance parameter g_m can be approximated from #GUID-120A230B-5D0F-4C30-99F3-77806C02CB36. At a drain-to-source current of 1 mA, the measured g_m is approximately 7.5 mS.

The DC A_ol of the OPA202 is 150 dB. Combined with the JFET gain, the resulting feedforward gain A is calculated as shown in #EQUATION-BLOCK_GLL_4BT_NVB. The simulated feed forward gain is shown in #FIG_YFV_2XS_NVB and is A = 175 dB. This closely matches the calculated result.

Because wafer process variations can yield up to 30% variations in g_m, adding a feedback network (β) maintains a predictable closed-loop gain. The β feedback network consists of resistors R_F, and R_G2 and is a series-shunt feedback network. The β network samples v_out by shunting the output of the OPA202 and feeds back a proportional voltage v_fb. The voltage v_fb = v_gs2. The gate is the feedback-summing node of the circuit. In this configuration, the loop is closed. An increase of the input differential voltage v_id results in a rise of v_od and therefore v_out. If v_out rises, then v_gs2 rises. An increase of v_gs2 lowers v_id. A decrease of the differential voltage between the JFET drains is observed. The final outcome is a reduction of v_out which completes the negative feedback loop of the preamplifier.

The standard closed-loop gain (A_cl) #EQUATION-BLOCK_MBQ_CCC_WQB applies.

Assuming the feedforward gain A is much greater than β, A_cl is approximately determined by resistors R_F and R_G2 in the mid-band frequencies. A_cl can be approximately calculated using #EQUATION-BLOCK_JJR_CFJ_MVB .

#FIG_CCJ_MTZ_5QB shows the closed-loop gain vs frequency response of the JFET preamplifier circuit.

#FIG_V2D_2HX_HWB compares the gain bandwidth of the JFE2140 composite preamp circuit to the OPA202 and the OPA145. Each configuration is in a gain of 60 dB. Table 2-1 shows that the JFE2140 composite preamp circuit achieves the highest bandwidth of the 3 circuits. This is made possible by the additional feedforward gain that the discrete JFET front end contributes. Table 2-1 also shows the ultra-low-noise performance that the JFE2140 composite circuit achieves.