Driver circuit

Abstract: A driver circuit for applying a first signal having a desired wave form, frequency and peak to peak voltage to a coaxial transmission line which also has applied to it a second signal having a substantially higher frequency in­ cludes an operational amplifier (26) having the first signal applied to its noninverting input (24). The output (28) of the operational amplifier is applied across the primary winding (34) of a driver coupling transformer (36) through an inductor (30) which provides high impedance to the second signal. The voltages induced in the secondary winding (38) of the driver transformer are applied to the transmission line (40). The primary winding (46) of a feedback transformer (48) is connected in parallel with the secondary winding (38) of the driver transformer. The voltage induced in the secondary winding of the feedback transformer (48) is applied by a feedback circuit (52, 54) including an R.C. filter (56, 58) to the inverting input (44) of the operational amplifier (26). The signal applied to the inverting input terminal includes both an A.C. component and a D.C. component, which compo­ nents cause the output current of the operational amplifier to induce in the secondary winding of the driver transformer a signal, the wave form, frequency and peak to peak voltage of which substantially equal those of the first signal applied to the noninverting input (24) of the operational amplifier.

Description: The invention relates to driver circuits for applying to a transmission line, the loading of which varies, a signal having a desired wave form, frequency and peak-to-peak voltage. US-A 44 53 131 describes a driver circuit, whereat the driver transformer includes a primary winding connected to the output of the amplifier, a first secondary winding connected to the load and a secondary winding used as a feedback winding, which via a feedback resistor is connected to the inverting input of the amplifier. Signals from a drive circuit are applied to the inverting and to the non-inverting inputs of the amplifier, and a DC feedback loop is provided between the amplifier output and the non-inverting input. This feedback circuit includes an operational amplifier, whose non-inverting input is connected to ground and whose inverting input is connected to the output of the drive amplifier by means of a resistor. A DC feedback network including a resistor and a capacitor connected in parallel is provided between the inverting input of the feedback amplifier and its output. This output of the feedback amplifier then is connected to the non-inverting input of the drive amplifier by means of a further feedback resistor. With this transformer coupled amplifier circuit transformer-induced non-linearities are eliminated by way of a dual DC/AC feedback configuration. DE-A 29 17 288 shows a feedback amplifier circuit including the features listed is the preamble of Claim 1 and comprising an operational amplifier having its non-inverting input connected to an input signal source and having its output connected via a power amplifier to the primary winding of a driver transformer. The secondary winding of said driver transformer feeds the load, and the primary winding of a feedback transformer is connected across the secondary winding of the driver transformer. The secondary winding of the feedback transformer is connected to a voltage divider whose tab is connected to the inverting input of the operational amplifier. This circuit achieves a very small output impedance and has a reliable frequency response. The use of coupling transformers to permit a driver circuit to apply signals to a transmission line is known in the art. The object of the driver circuit of this invention is to apply a relatively low frequency signals carrying binary coded information onto a transmission line which is simultaneously, or concurrently, carrying significantly higher frequency digital signals, without there being a conflict between the two signals. The environment in which this invention is to be used is a token passing local area network in which a limited, but significant number of physical modules, the number of which can change from time to time, communicate with one another over the local control network, transmission line, bus, or coaxial cable. The communication between physical modules is at a frequency of 5.0 MHz in the preferred embodiment. The peak to peak voltage of the higher frequency signals is limited in the preferred embodiment to 800 mV . In this environment one physical module is permitted, or enabled, to transmit timing information over one of the coaxial cables of the local network bus. The timing information is included in a timing frame prepared by the timing subsystem of the physcial modules so enabled. The timing frame includes information which permits timing subsystems of all physical modules of the network to synchronize themselves with that of the transmitter. When a redundant cable is used, a second module may be enabled to transmit timing frames over the second cable at a lower frequency 12.5 K Hz in the preferred emboidment. A physical module which is cable of transmitting timing frames over the local control network cable is provided with driving circuit of this invention. For additional information on this environment, reference is made to concurrently filed EP-A- 0 185 348 "Method and Apparatus for Synchronizing the Timing Subsystems of the Physical Modules of a Local Area Network". The disclosure of this EP application is hereby incorporated by reference into this application. The peak to peak voltage of the lower frequency signals applied to the cable or transmission line, has to be such that when it is added to the peak to peak voltage of the higher frequency signals, the sum of the two voltages is such as not to overdrive the receivers of the physical modules of the network. Controlling the peak to peak voltage is made more difficult because the load seen by the driver circuit is not necessarily constant. The wave form of the applied signal should not include harmonics of the higher frequency signals. If such harmonics were present, they could result in errors in the information transmitted at the higher frequency. In addition, the driver circuit has to provide high impedance to the higher frequency signals which are also carried by the cable to prevent overloading the driver circuit supplying the higher frequency signals. These and other objects are achieved by the invention as described in claim 1. The new driver circuit applies through a coupling transformer, signals on which strict limits are placed as to the wave form, peak to peak voltage, and a frequency of the signal as applied to the transmission line. Further details are characterized in the subclaims. The present invention provides a driver circuit which applies through the secondary winding of a driver coupling transformer a quasi-sinusoidal timing signal having a given peak to peak voltage and frequency to a coaxial cable. The operational amplifier of the driver circuit has inverting and noninverting inputs and produces an output which is applied through an inductance and which is capacitively coupled across the primary winding of the driver transformer. The inductance provides a high impedance to the high frequency signals present on the coaxial cable, but its impedance to the lower frequency timing signals is not significant. Signals applied to the driver circuit are a bipolar substantially square wave signal of a given frequency. The bipolar squarewave signal is filtered to remove higher frequency components, or harmonics and attentuated by an input circuit to apply a signal having the desired wave form, peak to peak voltage and frequency, to the noninverting input of the operational amplifier. The output of the operational amplifier is connected through the inductance and a D.C. feedback resistor to the inverting input of the operational amplifier. This circuit constitutes the D.C. feedback portion of the inverting input. The primary winding of the feedback transformer is connected in parallel with the secondary winding of the driver transformer. The voltages induced in the secondary winding of the feedback transformer are capacitively coupled to an R.C. filter. The filtered signal from this R.C. filter constitutes the A.C. feedback portion of the inverting input to the operational amplifier. The R.C. filter in the A.C. feedback circuit prevents the higher frequency signal on the coaxial cable from being included in the inverting input to the operational amplifier. The high frequency R.C. filter provides a relatively high impedance to the signal covered by the coaxial cable. The inverting input to the operational amplifier causes the signal applied to the coaxial cable to have the wave form, peak to peak voltage and frequency of the noninverting input of the operational amplifier. The circuit can apply signals to a transmission line, the wave form, peak to peak voltage and frequency of which satisfy predetermined criteria notwithstanding varying loads seen by the driver on the transmission line. Other objects, features and advantages of the invention will be readily apparent from the following description of certain preferred embodiments thereof, taken in conjunction with the accompanying drawings, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure, and in which: Figure 1 is a circuit diagram of the driver circuit of the invention; and Figure 2 illustrates wave forms utilized in the description of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENT In Figure 1 driver circuit 10 receives a square wave bipolar signal which is applied to input terminals 12 and 14 from a timing subsystem such as is described in the concurrently filed EP Application identified above. The wave form, peak to peak voltage and period of this timing signal is as illustrated in Figure 2 by wave forms 16 and 18. The timing signal applied to terminals 12 and 14 passes through input circuit 20 which attentuates and in addition filters out its higher frequency components to produce a signal having a wave form designated as quasi-sinusoidal with a peak to peak voltage as illustrated in Figure 2 by wave form 22. The quasi-sinusoidal output of input circuit 20 is connected to the noninverting input terminal 24 of operational amplifier 26. In the preferred embodiment, Op Amp 26 is a National Semiconductor LM13080 programmable power Op Amp. The output of Op Amp 26 at its output terminal 28 is applied through inductor 30 and capacitor 32 across the primary winding 34 of driver coupling transformer 36. Secondary winding 38 of transformer 36 is connected to coaxial cable 40 so that the voltages induced in secondary winding 38 are applied to coaxial cable 40, a transmission line, or local control network bus, of a token passing local area network, for example. The output terminal 28 of Op Amp 26 is also connected through inductor 30 and resistor 42 to the inverting input terminal 44 of Op Amp 26. The circuit including resistor 42 provides a D.C. feedback path to inverting input terminal 44. To provide an A.C. feedback to inverting terminal 44, the primary winding 46 of feedback transformer 48 is connected in parallel with the secondary winding 38 of transformer 36. The voltages induced across secondary winding 50 of transformer 48 are capacitively coupled through capacitor 52 and RC filter circuit 54 to inverting input terminal 44. Filter circuit 54 includes resistor 56 and capacitor 58. In the preferred embodiment, cable 40 a substantial percentage of the time is a high frequency signal having a peak to peak voltage of 800 mV at 5 M Hz. Driver circuit 10 drives a 12.5 K Hz signal having a peak to peak voltage of 95 mV onto cable 40. The wave form of the 12.5 K Hz is quasi-sinusoidal as the result of filtering out the high frequency harmonics of the Substantially square wave input signals applied to input terminals 12 and 14 by filter attentuator circuit 20. Circuit 20 also reduces the amplitude of the applied signal to that which is compatible with the system requirements, i.e., a peak to peak voltage at terminal 24 which does not exceed 100 mV in the preferred embodiment. Inductor 30, which is connected in series with output terminal 28 of Op Amp 26, allows the 12.5 K Hz output signals produced by Op Amp 26 to easily pass through and be applied across the primary winding 34 of driver transformer 36 while offering a high impedance to the 5 M Hz signal on cable 40. Resistor 42 in the D.C. feedback circuit also presents a high impedance to the 5 M Hz signal on cable 40. In order for driver circuit 10 to drive, or apply, signals onto cable 40, having the same wave form, peak to peak voltage, and frequency as those applied to noninverting input terminal 24 of Op Amp 26, it is necessary that the wave form, peak to peak voltage and frequency of the signal applied to inverting input terminal 44 of Op Amp 26 by the A.C. and D.C. feedback loops, or circuits, be the same as that applied to noninverting terminal 24. It might be assumed that the voltage across primary coil 34 of transformer coil 38 is substantially the same as those induced in the secondary winding 38. Such is not the case since the currents required to drive the many loads seen by secondary winding 38 cause losses in transformer 36 that result in the voltage across primary coil 34 being different from those across secondary coil 38. Thus, the wave form and voltage across winding 34 does not duplicate that across winding 38. To provide as the A.C. feedback signal one which duplicates that across winding 38 of driver transformer 36, primary coil 46 of feedback transformer 48 is connected in parallel with coil 38. As a result the wave form and voltage across the secondary winding 50 of feedback transformer 48 is a much more representative signal of that applied to cable 40. When this A.C. feedback signal is combined with the D.C. feedback signal through resistor 42 they cause the output of Op Amp 26 to be such that the wave form, peak to peak voltage, and frequency of the signal applied to cable 40 substantially matches in all respects, or duplicates the noninverting signal applied to terminal 24 of Op Amp 26. The circuit connected to pin 4 of Op Amp 26 is utilized to stabilize the gain of Op Amp 26. The circuit connected to pins 2 and 7 of Op Amp 26 prevent it from functioning when appropriate signals are applied to pin 60, which signals prevent Op Amp 26 from transmitting during power up and power down. From the foregoing it is believed obvious that the driver circuit of this invention applies signals to a transmission line, the wave form, peak to peak voltage and frequency of which satisfies predetermined criteria for these signals notwithstanding varying loads on the transmission line as seen by the driver circuit.

Claims: A driver circuit for applying a first AC signal to a load, said circuit comprising: a) an operational amplifier (26) having an inverting input (44) and a non-inverting input (24) and receiving said AC signal at one (24) of its inputs; b) a driver transformer (36) including a driver primary winding (34) which is in communication with the output (28) of said amplifier (26), and including a driver secondary winding (38) which is connected to said load (40); c) an AC feedback circuit (36, 38, 48, 56, 58) including a feedback transformer (48) having a feedback primary winding (46) which is connected across said driver secondary winding (38), and including a feedback secondary winding (50) which is coupled to the other input (44) of said amplifier (26); characterized in that d) said first AC signal is a low frequency quasi-sinusoidal signal having a substantially constant peak-to-peak voltage; e) said load is a transmission line (40) which concurrently carries a second higher frequency AC signal; f) impedance means (30), providing a high AC impedance to said higher frequency signals but providing a low impedance to said low frequency signals, is connected between said amplifier output (28) and said driver primary winding (34); g) said AC feedback circuit (48, 52, 54) includes a low pass filter (56, 58) for removing said high frequency signals from the AC feedback signal; h) a DC feedback circuit (42, 58) includes a resistor (42) connected with its one terminal to a connection line between said impedance means (30) and said driver primary winding (34), and connected with its other terminal to said other input (44) of said amplifier (26); whereat i) the components of said AC and DC feedback circuits (36, 38, 48, 56, 58; 42, 58) are adjusted such that the signal applied to the transmission line (40) substantially matches the low frequency signal applied to said one input (24) of the amplifier (26). The driver circuit of claim 1, characterized in that said impedance means (30) is an inductor. The driver circuit of claim 2, characterized in that a coupling capacitor (32) is connected between said inductor (30) and the primary winding (34) of said driver transformer (36) and said one terminal of said DC feedback resistor (42) is connected from the junction between said inductor (30) and said coupling capacitor (32) to said other input (44) of the amplifier (26). The driver circuit according to one of the claims 1 to 3, characterized by a capacitor (58) connected between said other input (44) and ground. The driver circuit of claim 3 or 4, characterized by a second coupling capacitor (52) connected between said feedback transformer secondary winding (50) and said other input (44) of said amplifier (26). The driver circuit according to one of the claims 1 to 5, characterized by an input circuit (12, 14, 20) including an input terminal (14), an output terminal (24) and at least one low pass filter (20), said input circuit adapted to have applied to it a substantially square wave low frequency signal having a given frequency and peak-to-peak voltage, the output terminal (24) being connected to said one input (24) of the amplifier (26) and providing said quasi-sinusoidal signal. The driver circuit according to claim 6, characterized in that a bipolar square wave signal having a peak-to-peak voltage of substantially 10 volts and a frequency of substantially 12.5 kHz is applied to the input circuit (12, 14, 20) and a 100 millivolt (mV) quasi-sinusoidal signal having a frequency of 12.5 kHz is applied to said one input (24) of the amplifier (26).

Inventors: SPIESMAN ROBERT L, SPIESMAN, ROBERT L.
Applicants: HONEYWELL INC, HONEYWELL INC.
Application year: 1985
Main classification: H03F1
IPC codes: H03F3, H04B3, H03B28