MULTIVIBRATOR CIRCUIT USED AS A DELAY ELEMENT

Abstract: The invention relates to a multivibrator circuit used as a delay element. The complementary signals to be delayed control the transistors (322,362) of a long tailed pair, so as to generate charging and discharging currents for a capacitance (38). The delay time depends among other parameters on the exact values of these currents. In order to avoid the necessity of very accurate values of the signals to be provided for generating the proper current values, a current division is applied, such that in parallel to each long tailed pair transistor (322,362) a constant current source (321,361) is added. Now the currents in the multivibrator circuit can be switched between two exactly determined values by alternately switching the long tailed pair transistors (322,362) fully on and off. A same kind of current division is applied in order to achieve the voltage swing at the output being independent on the current adjustment.

Description: The invention relates to a delay generator multivibrator circuit having a first and a second current branch providing delayed signal outputs, each of which comprises a series arrangement of a resistance, a main current path of a main transistor and a controllable current source, whereby the first connection nodes between the main current path and the controllable current source are coupled via a capacitance, and a control electrode of the main transistor, having its main current path inserted in the one current branch, is coupled to a second connection node between the resistance and the main current path of the main transistor in the other current branch, whereby cross-coupling of the main transistors is provided; whereby the controllable current sources are oppositewise controlled by the input signal to be delayed, thereby generating alternately in each of the current branches in a first state a first current and in a second state a second current. Such a multivibrator circuit is known from JP-Kokai 60-70804, where it is used as a delay element in a FM demodulation circuit. The controllable current sources in the known delay element are constituted by a long tailed transistor pair, the control electrodes thereof receiving mutually complementary signals to be delayed. The currents supplied by the controllable current sources in both the first and second state depend on the exact values of the input voltages, representing the input signals, on the control electrodes of the long tailed transistor pair. The ratio of these current determines the delay time attributed to the signals. It is an object of the invention to provide a multivibrator circuit in which the delay time is exactly determined in dependence on the input signals without the need for feeding the controllable current sources with input signals having accurately determined values. A way of adjusting the delay time in the prior art circuit is to adjust the total current, drawn by the controllable current sources. Then the problem arises, that the voltage swings across the resistances are changed. These voltages represent output signals to be fed to next circuits. Changes therein, induced by the adjustment, could be dealt with by buffers inserted between the multivibrator circuit and the next circuits. However, this increases the number of circuits. It is therefore a further object of the invention to provide a multivibrator circuit having a constant voltage swing across the resistances independent on the adjustment of the total current drawn by the controllable current sources. Therefore, a multivibrator circuit according to the invention is characterized in that in parallel to each controllable current source a constant current source is connected. The controllable current sources are being switched on and off instead of being switched between two non-zero-current states. In the first state a current in a current branch equals the constant current supplied by the constant current source. In the second state a current in a current branch then equals the sum of the constant current and the current supplied by the controllable current source. With this current division, the delay element is less susceptible to inaccuracies in amplitudes of signals controlling the controllable current source. Therefore, a broad variety of input signals may be offered at the input side, without affecting the output signals. With the same kind of current division the time delay caused by the multivibrator circuit in accordance with the invention is made adjustable without the need for adapting the circuits, fed by the multivibrator circuit, to the chosen adjustment. Therefore a multivibrator circuit according to the invention is characterized in that the controllable current sources together comprise a first current source and two further transistors, having control electrodes constituting control terminals for controlling the controllable current sources, and having their main current paths connected between the first connection nodes and the first current source, whereby there are provided a first and a second additional transistor, respectively, having a first and a second control electrode connected to the control electrode of the main transistor in the first and second current branch, respectively, and a first and second main current path connected to the second connection node in the first and second current branch, respectively, and to a second current source, whereby the sum of the currents of said first and second current sources is a constant. The two additional transistors connected to the second current source are switched in synchronism with the main transistors. The current through one of the resistances is divided into a first part conducted by the relevant additional transistor towards the second current source and into a second part, conducted by the relevant main transistor towards the capacitance and the relevant further transistor, constituting a component of the controllable current source. Both parts together generate the voltage across the resistance. The second part, charging the capacitance, gives rise to the delay time. By keeping the sum of the currents supplied by the first and second current source constant the voltage swing across the resistances is kept constant, which avoids adapting next stages to adjustments within the multivibrator circuit. The invention will be discussed below in the light of a drawing in which Figure 1 represents an embodiment of a multivibrator circuit in accordance with the invention, Figure 2 represents a further embodiment of a multivibrator circuit in accordance with the invention, Figure 3 represents a next further embodiment of a multivibrator circuit in accordance with the invention. In Figure 1 there is shown an embodiment of a multivibrator circuit in accordance with the invention. The circuit comprises a first and a second current branch respectively, which comprises a series arrangement of resistance 34, main transistor 35 and controllable current source 362/33 and a series arrangement of resistance 30, main transistor 31 and controllable current source 322/33, respectively. On the one hand the current branches are coupled via capacitance 38 and on the other hand via respective emitter followers 40/42 and 39/40, inserted between the control electrode of the main transistor in the one current branch and the connection between the resistance and the main transistor in the other current branch. The controllable current sources 362/33 and 322/33 constitute a long tailed pair, being fed via input terminals 3 and 4, which receive the input signal to be delayed. The delayed signal is present on output terminals 5 and 6. The delay time depends on the frequency of the input signal which must lie within a predetermined frequency range. When a constant voltage is applied on input terminals 3 and 4, the output signals on terminals 5 and 6 are opposite oriented oscillations, the amplitude of which depend on the induced currents in the current branches and the frequency of which in addition depends on the value of capacitance 38. When on the other hand the transistors 362 and 322 are alternately switched between the first and the second conducting state by applying varying input signals on terminals 3 and 4, the frequency of which lies within said frequency range, the delayed output signal follows the input signal. The delay time is determined by the current in the current branches and the value of capacitance 38. In the prior art multivibrator circuit the value of the input signals must be accurately determined in order to generate in the current branches currents having the proper values in the first and in the second state, thereby fixing the delay time. In the multivibrator circuit in accordance with the invention constant current sources 361 and 321 are connected in parallel to the controllable current sources in order to accomplish a current division. As transistors 362 and 322 may be alternately switched on and off, the currents in the current branches switch between two exactly defined values. In Figure 2 a further embodiment of a multivibrator circuit in accordance with the invention is shown. Like reference numerals designate identical or correspondings components as are shown in the previous figure. The circuit is extended by additional transistors 50 and 51 which are controlled in synchronism with main transistors 31 and 35. These additional transistors 50 and 51 enlarge the voltage swing across resistances 34 and 30 by drawing an additional current. Since the current through one resistance is divided into a first part drained by the relevant additional transistor e.g. 50 and into a second part conducted by the relevant main transistor e.g. 31 the enlargement of the voltage swing does not influence the capacitance charging current and therefore not the delay time determining quantities. The delay time is adjusted by adjusting the second part of the current conducted by transistors 362 and 322. However, changing this second part implies, without further measures taken, a change in the voltage swing across the resistances. Therefore, the sum of the total current, conducted by transistors 362 and 322, and by the additional transistors 50 and 51 must be kept constant. This is accomplished by connecting the long tailed pairs 50/51 and 362/322 to respective connections of long tailed pair 60/61 being controlled via terminals 63 and 64 and conducting a current supplied by current source 62. In Figure 3 a preferred embodiment of a multivibrator circuit in accordance with the invention is represented. Again, like reference numerals designate identical or corresponding components as are shown in the previous Figure. Now main transistors 31 and 35 and additional transistors 50 and 51 shown in the previous Figures have been replaced by two multi-emitter transistors 311 and 351. The long tailed pair in transistor arrangement 311/351 and long tailed pair 322/362 respectively, now are connected to current mirrors 72/73 and 70/71 respectively, which are controlled by the long tailed pair of control transistors 74/75, which has a current source 62 connected to supply terminal 37. As compared to the previous Figure, the advantage here consists in that less components are placed in series between the supply terminals. This implies that a lower supply voltage is sufficient to drive the circuit.

Claims: Delay generator multivibrator circuit having first and second current branches providing delayed signal outputs, each of which comprises a series arrangement of a resistance (30, 34), a main current path of a main transistor (31, 35) and a controllable current source (322, 362); whereby first connection nodes between the main transistors (31, 35) and controllable current sources (322, 362) are coupled by a capacitance (38), and a control electrode of the main transistor (31, 35) in the one current branch is coupled to a second connection node between the resistance (34, 30) and the main current path of the main transistor (35, 31) in the other current branch whereby cross-coupling of the main transistors (31, 35) is provided; whereby the controllable current sources (322, 362) are oppositewise controlled by the input signal to be delayed thereby generating alternately in each of the current branches in a first logical state a first current and in a second logical state a second current, characterized in that in parallel to each controllable current source a constant current source (321, 361) is connected. Multivibrator circuit as claimed in Claim 1 characterized in that the controllable current sources (322, 362) together comprise a first current source and two further transistors (322, 362) having control electrodes constituting control terminals for controlling the controllable current source, and having their main current paths connected between the first connection nodes and the first current source (62), (61; 70, 71, 74) and whereby there are provided a first and a second additional transistor (50, 51) respectively, having a first and second control electrode connected to the control electrode of the main transistor (31, 35) in the first and second current branch, respectively, and a first and second main current path connected to the second connection node in the first and second current branch respectively, and to a second current source (62), (60; 72, 73, 75) whereby the sum of the currents of said first and second current sources is a constant. Multivibrator circuit as claimed in Claim 2, characterized in that the first (61, 62) and second (60, 62) current source together comprise a long tailed pair of a first (61, 74) and a second (60; 75) control transistor, said pair being connected via a third current source (62) to a supply voltage terminal, the first and second current sources being oppositely controlled via the control electrodes of said first and second control transistor. Multivibrator circuit as claimed in Claim 3, characterized in that the further transistors (322, 362) and the additional transistors are connected to a first (70, 71) and a second (72, 73) current mirror respectively, which are controlled by the first and second control transistor respectively the first (74) and second (75) control transistor being of a conductivity type different from that of the further (322, 362) and additional transistors.

Inventors: GEHRT, HOLGER, HILDEBRANDT, GUENTER, REHFELDT, KARL-HEINZ, Hildebrandt, Günter
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN, PHILIPS PATENTVERWALTUNG GMBH
Application year: 1988
Main classification: H03K3
IPC codes: H03K3