JPH0758866B2 - Control pulse amplifier for VTR - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an amplifier, and more particularly to an amplifier having excellent transient characteristics.

[Background of the Invention]

Conventional small-signal amplifiers have not sufficiently taken into consideration the transient characteristics in which the input signal changes from large to small. That is, when the large signal input is switched to the small signal input, the bias of the amplifier largely deviates from a predetermined value and the transistor is turned off. After that, it takes a lot of time for the bias to return to a predetermined value. Related to this type of circuit is, for example, "Basics of Solid State Amplifier" (pages 182 to 185) issued by Radio Technology Co., Ltd.

The conventional technique will be described in detail with reference to FIG. In the figure, 1,2,
3,4,5,7 to 12 are resistors, 20 to 24 are capacitors, 30 is a control head, and 33,35,36 are transistors. This amplifier is configured as a three-stage direct-connection amplifier, the first stage and the third stage are grounded emitter amplifiers, and the second stage is an emitter follower for impedance conversion. The resistor 9 (resistance value R9) is a resistor for AC feedback, and if the oven-loop gain of the three-stage direct-coupled amplifier is A ₀ , the closed-loop gain A _c is However, Becomes And when A ₀ >> 1, it becomes A _c 1 + R ₉ / R ₁₁ .

The resistor 10 (resistance value R ₁₀ ) is a DC feedback resistor and determines the base potential of the input transistor 33.

FIG. 3 shows an example when used as a control pulse amplifier of a VTR.

In this amplifier, during reproduction, a control pulse (CTLP) of about 1 mV reproduced from the control head 30 is input and amplified. During recording, a control pulse (REC / CTLP) for recording is supplied to the control head 30 through the resistor 12 and the capacitor 24 for recording.

Now, when considering the joint shooting operation, the mode changes from record to record pose to record pose.
At this time, in order to match the joint, record (REC)
When a record pause (REC / PAUSE) occurs, the recorded part is played in reverse, and after a certain amount of reverse, REC / PAU
SE status. After releasing the Pomose, the control pulse and the vertical synchronizing signal are phase-aligned by the assemble operation, and after a certain amount of phase alignment, a new recording state is set. By this operation, the joining portion is joined without noise generation.

In the above, when the time for the fixed amount of reverse reproduction is determined by the number of control pulses (CTLP), the input of this control pulse amplifier is a large amplitude input at REC (almost 0 to V _CC is input). , A small amplitude of about 1 mV is input at the moment of reverse playback.

As shown in the figure, since the capacitor 20 is connected to the input of this amplifier, the potential difference between both ends is kept constant due to the characteristic of this capacitor, and when switching from the large amplitude input to the small amplitude input, the transistor 33 The base potential is also displaced by the change amount, and the bias of each part of the amplifier is disturbed, so that the amplification operation is not performed. If the change over time of the base potential of the transistor 33 is described, Becomes However, ▲ V ⁽⁰⁾ _B33 ▼ is the transistor 33 in steady state.
The base voltage of ▲ V ⁽⁰⁾ _E36 ▼ is the transistor 36 in the steady state.
The emitter voltage of V _CC and V _CC correspond to the high level of the control pulse for recording.

From this formula, the potential difference V _CC −V _BO 5−0.7 = 4.3 volts is τ
= By discharging a time constant of C ₂₀ R _10, it takes much time to amplifier returns to normal operation. Therefore, there is a drawback in that the reverse reproduction amount is deviated. In short, in the conventional amplifier, when the input amplitude changes, the operating point of the circuit largely deviates from a predetermined value, and when the bias is restored to its original value through a time constant circuit, the recovery time is large. It becomes large and the transient characteristic becomes bad.

[Object of the Invention]

The object of the present invention is to eliminate the above-mentioned drawbacks of the prior art and, when switching from a large-amplitude input to a small-amplitude input, the bias of each part can be immediately stabilized to a predetermined value and exhibit a desired amplification effect. To provide an amplifier.

[Outline of Invention]

In order to achieve this object, the present invention has a short time constant by connecting a voltage limiting element across a resistor forming a DC feedback circuit and showing a high resistance in a steady state and a low resistance in a transient state. Thus, the feature is that the recovery time is shortened.

Example of Invention

 Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of an amplifier according to the present invention, wherein 1 to 12 are resistors, 20 to 24 are capacitors, 30 is a control head, and 31 to 37 are transistors.

In the figure, the overall configuration of the amplifier will be described. The first stage is a differential amplifier including transistors 33 and 34 and active loads 31 and 2, the second stage is an emitter follower circuit using a transistor 35, and the third stage is a transistor 36. Is a grounded emitter circuit. That is, this amplifier is a three-stage direct-coupled amplifier circuit. The resistor 9 is an AC feedback resistor, and the total closed loop gain is However, R _6: the resistance of the resistor 6 values R _9: the resistance value of the resistor 9.

The resistor 10 is a DC feedback resistor.

The resistor 1 and the capacitor 21 form a low-pass filter, and remove unnecessary signal components in the high band. The open loop gain of the amplifier shown in FIG. 1 is several tens of dB in the first stage amplifier and several tens of dB in the third stage grounded-emitter circuit.

Now, consider the case where a signal such as υ _in of FIG. 2 is input to the circuit of FIG. It is a rectangular wave with a time width t ₁ of υ _in , and we are considering a large amplitude input. Waveforms A, B, C and υ _{out in} Fig. 2
Shows the waveform of each part in FIG.

When the large amplitude input is switched to the small amplitude input at time t ₁ , the potential difference across the capacitor 20 is substantially constant, so the base potential of the transistor 33 changes as shown in FIG. 2A.
In this case, since there is the transistor 37 which is diode-connected in parallel with the resistor 10 (resistance value R ₁₀ ), the potential difference across the resistor 10 does not exceed 0.7 volt. That is, the base potential of the transistor 33 does not drop more than 0.7 volt due to the emitter potential of the transistor 36. Therefore, A
The potential of is given by the following equation.

From this equation, the potential at the point A does not drop much compared to the configuration without the diode (diode-connected transistor 37), and the transistor 33 starts its operation at an early stage. That is, the amplifier has excellent transient characteristics.
FIG. 2 shows the time responses of points B, C and υ _out .

As described above, according to this embodiment, it is possible to configure an amplifier having excellent transient characteristics and improve performance. If this circuit is used as a VTR control pulse amplifier, it is effective for joint shooting.

〔The invention's effect〕

As described above, according to the present invention, even when the input signal level changes from a large level to a small level, the direct current bias can stably perform the amplifying action, that is, the transient characteristic is excellent, and It is possible to improve the performance, and to provide an amplifier having an excellent function, excluding the above-mentioned drawbacks of the prior art.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of an amplifier according to the present invention,
FIG. 2 is a waveform diagram showing the main waveforms of FIG. 1, and FIG. 3 is a circuit diagram showing a conventional example. 1 to 12 ... resistance, 20 to 24 ... capacitor 31 to 36 ... transistor 37 ... diode-connected transistor (voltage limiting element)

Claims (2)

[Claims] 1. A VTR control pulse amplifier in which a VTR control pulse signal is input to the base of a transistor by capacitor coupling and the base of the transistor is negatively fed back by a DC negative feedback resistor. A VTR control pulse amplifier, characterized in that a diode connected in parallel with the resistor to limit the voltage across the DC negative feedback resistor to within a fixed value is provided only in the forward direction toward the base of the transistor. . 2. The VTR control pulse amplifier according to claim 1, wherein a diode-connected transistor is used as the diode.