J. Phys. E: zyxwvutsrqpo Sei. Instrum. 21 (1988) 667-673. Printed in the UK zyxwvutsr A high-voltage electro-optic driver with DMOS power FETS and optocoupling isolation N Theophanous, S Tsitomeneas, G Alexakis, A Arapoyianni and G Papaioannou Athens University. Physics Department. Panepistimioupoli, Ktiria TYPA, 157 71 Athens. Greece Received 14 September 1987, in final form 29 December 1987 Abstract. A new linear high-voltage driver capable of delivering floating AC voltages in the kV range for the normal type electro-optic loads is presented. The system‘s novelty lies in the utilisation of multiple amplifying channels using mios power MOSFET pairs isolated by means of LEdphotodiode optocouplers. General design criteria and justifications are first considered, along with the circuitry of the various stages. An implemented system is presented, which has proved capable of driving a 5 pF electro-optic crystal load over a zyxwvutsrq 5 Hz-2.0 MHz frequency range or a 62 pF Kerr cell up to 180 kHz, with a maximum distortion of zyxwvutsrqpo 2% for a voltage up to 1.5 kV peak to peak. Gain-bandwidth products of 4.0 GHz and 350 MHz, respectively, have been measured while noise and offset performance were satisfactory. 1. Introduction In several fields of applied physics it is very helpful to use a wide-band high-voltage driver providing an AC output up to the order of 1 kV peak to peak. Such a driver is particularly useful in studying electro-optic (EO) effects, which usually require voltages in the kV range and are more accurately investigated when using dynamic or AC methods of measure- ment (Carpenter 1950, Wilson and Hawkes 1983). Also, the availability of an efficient high-voltage driver broadens the selection chart of materials suitable for EO modulators or OE processing devices. In fact, most of the current materials that exhibit strong EO responsivity, such as KDP. ADP and perovskite-type crystals. present thermal instabilities and sig- nificant dielectric losses. By contrast, other materials, such as GMo-type crystals or Kerr effect liquids: although having poor electro-opticity, offer improved EO ‘figures of merit’ and stability advantages (Hartfield and Thompson 1978. Kaminow 1974, Theophanous 1976). Obviously, the latter may constitute satisfactory EO modulators if they are driven by high-voltage EO drivers. An efficient EO driver must have general advantages such as increased linearity and reduced distortions. sufficiently large bandwidth, low noise and offset voltages, etc. Additionally, a good quality EO driver, especially a high- voltage one, ought to possess a number of independent floating outputs, without common ground, being in phase and separately controlled and adjusted. This allows not only an increase total output voltage, obtained by additive superposi- tion in series of the partial outputs, but also various combi- nations of these outputs on multiple EO loads. Indeed, in several applications, composite EO modulators or processing systems are used that employ specific multiple-crystal confi- gurations demanding simultaneous driving by a number of floating outputs (An zyxwv et zyxwv a1 1976, Enscoe and Kocka 1984. Hartfield and Thompson 1978). Moreover, the above multi- floating character of the driver outputs allows their intercon- nection with various external DC voltages for EO biasing. This is particularly advantageous for the proper selection and control of the operating point of a composite EO modulator or light stabiliser (Enscoe and Kocka 1984). The high-voltage drivers currently available fail to meet the above requirements simultaneously and they are usually expensive. power consuming and cumbersome. Aiming to cope with this problem and reduce drawbacks, we have conceived and implemented the new high-voltage EO driver described herein. This driver is based on the application of a particular parallel optocoupling isolation circuitry to an arrangement of channels/modules using an improved high- voltage DMOS amplifier recently developed by the authors (Theofanous et a1 1987). In the optocoupling stages specific techniques are used to enhance linearity and bandwidth. The experimental prototype of the system to be presented is a twin-channel driver providing two individual floating outputs, each up to 780V peak to peak, which are independently controlled and isolated by means of separate Ledphotodiode paths. The entire system has an overall gain of about 2000 and its combined (total) linear output ranges up to 1.58 kV peak to peak with low distortion (<2%). satisfactory bandwidth (2.0 MHz for an EO load of 5 pF), negligible offset and a quite acceptable noise level (maximum SNR=~~ dB). The system has some rather minor imperfections which are also con- sidered in the paper. 2. General topology of the system A simplified block diagram of the system, in its general form, is depicted in figure 1. As shown, the system is composed of a number of similar channels (modules), M,, zy M:, . . . , M,,, that are independently optocoupled to a LED driver stage, (LDS), which is common to all channels and driven by a single input signal generator (ISG). In each channel, the output amplifier stage (oAs) consists of a DMOS low-current differen- tial push-pull amplifier (LCDPA) differentially driven by a dual operational amplifier phase splitter (PS).This splitter receives the output of a photodiode preamplifier (PDrA) whose photo- diode (PD)is optically coupled to its respective LED (LD). thus forming a discrete-component optocoupler (oc). The LEDS of all the channels are connected in parallel and the resulting shunt network loads the LED driver (LDS). Also, for better isolation, individual power supply units (PSU) are used separ- ately for each channel and for the LDS driver. Note that the optocoupling isolation. in comparison with the magnetic one, presents important advantages, such as greater isolation, wider and flat frequency response, lower cost, greater control facilities and absence of electromagnetic interference (Olschewski 1976). Nevertheless, optocoupling has the disadvantage of the existence of increased distortions due to LED nonlinearities. To cope with this drawback and further extend the bandwidth, while gaining control of LED/PD spacing and orientation. each oc has been built with discrete LED and PD components driven by specifically designed LDS and PDPA circuits, as described below. 3. The LED driver stage (LDS) It is well known that in analogue transmission optocoupling systems, such as the present one, the signal integrity is principally injured by the signal-to-light nonlinearities of the LED source, while the system’s photodiode comparatively behaves as a very linear element (Goodfellow and Davis 1984, Shumate and DiDomenico 1980). Therefore, our major preoccupation in the design of the LDS was the improvement 0022-3735188/070667 + 07 $02.50 @ 1988 IOP Publishing Ltd 667