4× JFET Buffer Amplifier Cuts Noise in Half

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Chau Tran

In many electronic circuits, there is always a demand for a device to isolate or separate one circuit from another. This special device is called a buffer. A buffer is a unity-gain amplifier that has an extremely high input resistance and an extremely low output resistance. This means that the buffer can be modeled as a voltage controlled voltage source that has a gain of one. Since the buffer ideally has an infinite input resistance, there is no loading effect, so that VIN = VOUT. Furthermore, the output voltage from the buffer is insensitive to the load resistance because the idealized buffer has an output resistance that is essentially zero. By placing a unity-gain buffer between a digital-to-analog converter (DAC) and a load, one can easily solve the loading problem.

When adding a unity-gain buffer to a system, it is important to maintain accuracy and performance. The most important consideration is to calculate the added noise:

Where:

en = buffer input voltage noise density
in = buffer input current noise density
f = device input bandwidth (Hz)

In the circuit of Figure 1, each channel has extremely low current noise (0.8 fA/√Hz) in comparison with voltage noise of (13 nV/√Hz). Therefore, when one needs less added noise in the system, it is critical to reduce this voltage noise. The voltage noise can be reduced by placing multiple buffers in parallel. For example, two buffers in parallel reduce the voltage noise by √2, or all four buffers placed in parallel act as a buffer with ½ the noise. The trade-offs to this method are increased bias current, current noise, and input capacitance but in this case, those results are negligible. Place a small resistor, such as 50 Ω, between the outputs to avoid extra current flow due to the slight differences between each output. For less power sensitive applications, these 50 Ωresistors can be omitted to boost the available output current.

The circuit of Figure 1 is a new configuration of a buffer amplifier that reduces the voltage noise by a half. The AD8244 is a quad, JFET input, unity-gain buffer that is designed to exceed expectations. The 2 pA maximum bias current, near zero current noise, and 10 TΩinput impedance introduce almost no error, even with source impedances well into the megaohms. With its low voltage noise, wide supply range, and high precision, this device is also flexible enough to provide high performance anywhere a unity-gain buffer is needed, even with low source resistance.

The Figure 2 plot is the comparison of the noise performance of a normal single-channel buffer and the new AD8244 buffer utilizing four channels in parallel.