Electronic Design

Single-ended to differential twisted-pair driver

The proliferation of personal computers and particularly of video applications has created a need for sending high-speed analog signals over twisted pair for short distances (up to 200 feet). Twisted-pair data transmission is easy to set up and the wire cost is very low compared to coax, so it’s becoming a popular replacement for coax. Most electronic signals exist in a single-ended format, so the signal must be converted to a double-ended or differential format to take advantage of twisted-pair datatransmission schemes. The circuit described here converts single-ended analog or digital signals into a differential signal capable of directly driving a twisted pair cable (Fig. 1).

Q1, Q2, and Q3 are one half of a HFA3102 dual long-tailed-pair transistor array, and are configured in to function as a linear differential amplifier. Because the transistors are matched, they will yield nearly identical performance depending upon their bias circuits. The base of Q2 is biased at 1.24 V, and the base voltage of Q1 ranges from 1.0 to 1.6 V depending on the setting of R3. When R3 is set at 1.24 V, the signals are amplified equally by both transistors. Consequently, R3 functions as a symmetry adjustment that can be used to obtain equal amplitude but opposite phase outputs at the collectors of Q1 and Q2. This criteria satisfies the definition of a differential signal.

The differential gain is 5 to 7, as configured in Fig. 1. The gain is set in this range because a typical video signal is less than 2 V in amplitude, and these low gains won’t cause distortion. R5 adjusts the current through both transistors and, since the gain is proportional to the emitter current, R5 functions as a gain control. If the gain range is too high, the inverting inputs (pins 3 and 6) of the HFA1212 programmable-gain amplifiers can be floated, making the gain fall to half of its previous value. If higher gains are required because longer twisted pairs must be driven, the differential output has to be fed into a transformer which then drives the twisted-pair cable. The transformer drive increases the signal amplitude without introducing distortion.

The circuit as shown will drive twisted pair cables directly. Each cable wire is connected to one of the differential outputs, and the cable is terminated at the receiving end in its characteristic impedance (about 100 Ω).

The frequency response for linear signals shows a −3-dB bandwidth of 200 MHz (Fig. 2). The response of twisted pair cables falls off at higher frequencies, so the amplifier response curve is purposely peaked at higher frequencies to compensate for this effect. This combination will yield a flatter overall frequency-response curve.

This driver, when coupled with a differential receiver, will reliably transmit data over 200 feet of twisted-pair cable in the presence of several hundred millivolts of single-ended noise. Digital signals should be handled the same as video signals, except that wide fluctuations in the digital data rate will cause skew due to the ac coupling.

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