High-voltage current-feedback amplifier is speedy

Joseph Ting, Institute of Atomic and Molecular Sciences of the Academia Sinica, Taipei, Taiwan -- EDN, 4/26/2001

The circuit in Figure 1 powers a microparticle and nanoparticle ion trap through a 1-to-5-turns-ratio, high-voltage transformer. It also works successfully as a driver for a piezo-tube scanner and in a near-field scanning optical microscope. The circuit is robust and works with supplies ranging from ±50 to ±230V. The measured parameters at ±230V supply voltage are gain of 26-dB from dc to –3-dB point at 7 MHz; output swing of ±200V, rise and fall times of 70 nsec for an output step of 350V, slew rate of 4100V/µsec, and supply current of 56 mA.

The red LEDs, D₁ and D₂, in Figure 1 provide a 1.8V drop; the LEDs are more rugged than precision IC voltage references. The current supply for IC₁ comes from R₁ and the source comprising D₁, R₂, R₃, and Q₁. R₃'s trimmed value is such that Q₂'s quiescent current is approximately 15 mA. You can determine this current by measuring the voltage drop across R₄. The same adjustment also controls the output-voltage offset. IC₂ is a unity-gain, high-current driver for Q₂. D₃ prevents IC₂'s input from going more negative than its negative supply. Q₃, D₄, C₁, and R₅ provide the negative bias for IC₂. Q₄ is an output-current limiting switch. Q₄ starts to turn on at I_OUT=290 mA. You can replace the bipolar transistors C3955 (npn, Q₂ and Q₆) and A138 (pnp, Q₃ and Q₇) by equivalents as long as they have the following minimum specs: V_CEO250V; I_C100 mA, and f_T100 MHz.

You should mount all the power transistors in individual finned heat sinks with an overhead 3-in. fan for cooling. The pc-board layout is not critical and needs no ground plane. However, you must use single-point grounding to minimize ringing. For the component values shown, the circuit is very stable and needs no compensation capacitors. Figure 2 shows a large-signal response for a ±9V, 1-MHz square-wave input. This circuit has a fixed gain of 20. For higher gains, you can increase the values of R₆ and R₇. For lower values, it is better to insert an attenuator at the input, because smaller values of R₆ and R₇ may result in excessive dissipation. Do not change the value of R₈, because it is optimized for speed. Be cautious when measuring and using this circuit, because it harbors lethal voltages. The National Science Council of Taiwan sponsored this project.