DE969246C - Unpolarized pulse switch working with flat transistors according to the principle of a bistable toggle arrangement - Google Patents

Description

The principle of the non-polarized pulse switch »with Surface transistors resembles that of the arrangement built up with electron tubes and is known.

If so, the dimensioning of the pulse switch is independent of the sample variations of the transistors If the two stable operating states are always to be guaranteed reliably, one must a) the voltage between the base and emitter of the blocked transistor large enough for the most unfavorable transistor combination make and b) let the switched-on transistor work in the saturation region.

Condition a) requires a high control voltage, condition b) results in a low one Input resistance, so both a low sensitivity of the switch.

If you place the operating point of the current-carrying transistor in the active area, you get a relatively high input resistance at this transistor, but you have to stabilize this operating point because of the sample variations. This is done by introducing a negative feedback resistor Rg in the common emitter line and investing a

80 »511/2 +

Cross current through R%, R _x and R _B , which must be much larger than the base current of the transistor when switched on (Fig. I).

The negative feedback resistor R _E reduces the gain of the current-carrying transistor and thus reduces the sensitivity of the switch. This disadvantage is countered by the introduction of a very large capacitor C _E , which makes the resistor R _E ineffective for the switching time ίο.

The voltage drop across R _E depends in the final state on the transistor that is currently switched on. In the case of two different transistors, the voltage at R _{E is} different in the two stable operating states of the switch. If the switching frequency is high enough, this voltage is directed, because of the capacitor C _E, to the transistor which, when switched on, carries the larger emitter current. This voltage can now be so great that the other stable operating state of the switch, which requires a different voltage between the emitter and zero point, only becomes stable when the capacitor C _{E has} been reloaded to this other voltage. This occurs particularly when, in view of the high sensitivity of the switch, the voltage difference at the base between the switched-on and the switched-off state of the transistor has been selected to be very small. The switching speed is then very small, corresponding to the time constants R _E C _E. The capacitors, C _x are used to increase the sensitivity by increasing the gain of the control pulse from one base via the collector to the other base.

In contrast, the invention (Fig. 2) firstly uses a divided collector resistance (R _ki + Rk ₂ ), through which, even with a large cross _current through R k2, R _x , R _B and a small voltage difference at the base, between the switched-on and the switched-off state of the transistor allows a large useful pulse to be achieved at the collector, secondly, the circuit of the capacitor C _x not only to bridge R _x , but between the collector and the base, so that the new state is achieved even with very small voltage differences set at the base when switching over, although the voltage on the emitter is still the old one. Has value, thirdly, the choice of the time constant C _E R _E and the time constants formed from C _x and the resistor combination in connection with the switching time of the transistor so that during the switching time a) the excessive voltage arises through C _x at the base, which safely initiates the new state, b) the voltage at R _E can change to its new value in the time in which the excessive voltage is at the base.

In order to keep the input conductance of the pulse switch as small as possible, one will have to make C _{x as small as possible.} The limit is given by the sound time of the transistor, which is essentially due to the frequency dependence of the current amplification

α =

is determined. Using the T equivalent image of the transistor (Figure 3), a value for C _x that is sufficiently large for all cases results:

C _x =

(r _b + r.) (Rk 2 + R _x )

wg (i - O ₀ ) (r _b + ) [R _K 2 (R _x +

where wg is the limit circle frequencies. 2. of the transistor in bias circuit and ot _{o is} the current gain at low frequencies. The size of the capacitor C _E should then be between the following limits:

C _E <

wg (1 - O ₀ )

and

wg (1 - a ₀ ) R _E

+ -

ι -

Figure 4 shows an example of dimensioning.

The advantage of the new circuit over the previously known is that it is a pulse switch of great sensitivity and short switching time can be built without regard to the manufacturing tolerances of the transistors. So it will be particularly advantageous where pulse switches used in large numbers and therefore mass-produced, such. B. at Number calculators and similar devices.

Claims (1)

PATENT CLAIM: Not polarized, with flat transistors based on the principle of a bistable flip-flop arrangement Working pulse switch of great stability H5 and input sensitivity, with which for stabilization of the operating point in the active area of the transistor and a capacitively bridged one Negative feedback resistance is provided in the common emitter line, as well as an opposite to the base current of the transistors large cross current is passed through the base resistors, characterized in that the Coupling resistances from the collector output of one transistor to the base input of the other transistor placed on taps of the collector resistors are, these are subdivided in such a way that even with a large cross current through the coupling resistors a large useful pulse at the collector arises that further between the collector of the one and the base of the other transistor capacitors are placed, which are dimensioned are that. the resulting time constant in connection with the time constant in the emitter circuit and the switching time of the transistor type used causes an excessive Tension arises, which safely introduces the new operating state and subsides in a time in which the voltage at the negative feedback resistor changes to its new value. For this purpose ι sheet of drawings © 6 »578/163 7.56 (809511/24 5.58)