JPS6232523B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an integrated circuit suitable for a head amplifier of a video tape recorder.

Figure 1 shows a head amplifier circuit using current integrated circuits. The problem with this circuit is that the variable resistor 7
When damping is adjusted, not only the peaking amount but also the peaking frequency changes, and the gain of the amplifier changes significantly due to the damping adjustment, so an external resistor 9 is required to reduce this change. It is to be. Below, the causes of the above problems and the operation of the conventional integrated circuit will be explained.

The integrated circuit 4 shown in FIG. 1 has been developed with sufficient sophistication in terms of the number of elements and the number of external terminals.

That is, in order to keep the collector current of transistor Q ₁ constant, a DC feedback terminal 8 is provided, and thereby the collector potential of transistor Q ₂ is set to (3+
R5/R6) Vbe, and the direct current potential generated at terminal 8 is used as the reference potential of the differential amplifier of transistors _Q5 and _Q6 to realize a direct-coupled high gain amplifier. Note that R ₅ and R ₆ are the resistance values of the resistors 5 and 6, and Vbe is the base-emitter voltage of the transistor. Furthermore, a variable resistor 7 is connected to this terminal 8, making it possible to adjust the amount of peaking in head peaking.

Head peaking is the process of causing the inductance and capacitance of the video head 1 to resonate to provide frequency characteristics as shown in FIG. Peaking frequency ₀ in the graph is trimmer capacitor 3
to the desired value (near the highest frequency of the playback FM carrier)
Approximately 4.5M for a VHS video tape recorder
Hz), and the peaking amount Q is set by variable resistor 7.
Similarly, it is set to the desired value (approximately 10 dB for VHS systems). This damping adjustment adjusts the input impedance of the amplifier by changing the feedback gain β of negative feedback.

Now, if the gain from the base of the transistor Q ₁ to the external terminal 8 is β, the input impedance of the amplifier is ZinR ₉ /1+β. In addition, Zin is the input impedance of transistor _Q1 , _R9 is resistor 9
, and if R ₉ = 10kΩ, the above-mentioned Q
= approximately 10 dB requires β≒20.

The problem here is that if β = 20, the base of transistor Q ₂ also has the input of transistor Q ₁ .
A 20 times higher signal appears and the high frequency gain drops due to the mirror effect of transistor _Q1 . This is because when an attempt is made to reduce the peaking amount Q with the variable resistor 7, β increases and the peaking frequency at the output decreases. This also causes the inconvenience that the trimmer capacitor 3 must be readjusted.

Another problem is that an external resistor 9 is required. The reason this is needed is transistor Q ₁
This is because the output of transistor _Q4 is voltage-divided by resistors 5 and 6 as a method of applying a DC bias to the base of Q4.

In other words, when trying to adjust damping so that Q = 10 dB using only resistor 10, the feedback gain needs to be (R5 + R6/R6) times as large as β when using resistor ₉ (R ₉ ). The gain of the differential amplifier of transistors Q ₅ and Q ₆ decreases,
This causes a problem in that the gain of the differential amplifier changes greatly due to damping adjustment. SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art and to provide an integrated circuit for a head amplifier in which changes in peaking frequency and changes in gain due to damping adjustment are extremely small. In the present invention, the second emitter follower
Using a diode-based DC level shift circuit, the output signal of Q ₄ is passed through the second resistor 10 to the first stage transistor without attenuating the AC signal.
A feature is that in addition to feeding back to the base of _Q1 , a DC bias independent of the feedback circuit is provided at the base of transistor _Q2 of the cascode amplifier.

FIG. 3 shows an embodiment of a head amplifier for a video tape recorder using the integrated circuit of the present invention. In the figure, Q ₃ is the first emitter follower, Q ₄ is the second emitter follower, 11 is the first resistor,
10 is a second resistor. In this circuit, the DC potential of the collector of transistor Q ₂ is connected to ground to ensure stable operation of transistors Q ₁ and Q ₂ .
5Vbe, and the base potential of transistor _Q2 is set to 3Vbe. Transistor to do like this
After the output of _Q4 is DC level shifted by 2Vbe using two diodes, it is fed back to the base of transistor _Q1 via resistor 10 ( _R10 ). In this way, the base potential of the transistor _Q1 becomes approximately Vbe regardless of the flowing current, so the output DC potential of the transistor _Q4 becomes 3Vbe. Further, the collector potential of the transistor _Q2 is 5Vbe, so the base potential of the transistor _Q2 may be 3 to 4Vbe.

The input potential of the transistors Q ₆ and Q ₇ is preferably 3 Vbe or more because the constant current circuit 14 is used.
If you want to set the input potential to 4Vbe, install three diodes at the output of transistor _Q4 , and set the input potential to 3Vbe.
All you have to do is level shift.

In the case of the constants shown in Fig. 3, the transistors Q ₁ , Q ₂
The gain of the cascode amplifier consisting of the following is about 40 dB, and when the resistance value of the variable resistor 7 is changed from zero to the maximum, the peaking amount Q changes by about 6 dB. At this time,
The differential amplifier gain of transistors Q ₆ and Q ₇ changes only by about 1.5 dB, which is no problem.

Further, the base bias of the transistor _Q2 is independent from the damping adjustment circuit, so that the AC signal generated at the terminal 8 does not appear at the base of the transistor _Q2 , and the peaking frequency does not change due to the damping adjustment. The present invention is also effective when the resistor 7 is a fixed resistor with a large value and sufficient feedback is applied to reduce the amount of peaking in the head peaking, thereby absorbing variations in the inductance value and Q of the head itself. That is, in the configuration shown in FIG. 1, the frequency characteristics deteriorate due to the above-mentioned reasons, and the external resistor 9 is required to compensate for the decrease in the differential amplifier gain of the transistors Q ₅ and Q ₆ .

By using the present invention, the head peaking circuit can be damped without requiring an external resistor 9 (R ₉ ), and the peaking frequency can be reduced due to this damping, and the differential voltage between transistors Q ₅ and Q ₆ can be reduced. The gain reduction of the amplifier can be reduced.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an example of a video head amplifier using a conventional integrated circuit, Fig. 2 is a graph showing the frequency characteristics of a head peaking circuit, and Fig. 3 is a video head amplifier using an integrated circuit of the present invention. FIG. 2 is a circuit diagram showing one embodiment of the present invention. 4... Integrated circuit, 8... External terminal, 10... Second resistor, 11... First resistor, 12... First emitter follower, 13... Second emitter follower.

Claims (1)

[Scope of Claims] 1. A cascode-type first-stage amplifier composed of a common-emitter transistor and a common-base transistor, a circuit for negatively feeding back an output signal of the common-base transistor to the base of the common-emitter transistor, and In a video head amplifier comprising a video head connected to the base and means for damping a resonant circuit generated at the base of the common emitter transistor,
an emitter follower whose input signal is an output signal of the common base transistor; a first resistor having one end connected to the emitter of the emitter follower;
A series circuit of a capacitor and a second resistor connected between the other end of the first resistor and ground, and a third resistor having one end connected to the base of the emitter-grounded transistor. , the other end of the third resistor and the other end of the first resistor are connected. 2. The video head amplifier according to claim 1, wherein the second resistor is a variable resistor. 3. The video head amplifier according to claim 1, wherein the emitter-grounded transistor;
The common base transistor, the emitter follower, the first resistor, and the third resistor are configured by an integrated circuit formed on the same substrate,
A video head amplifier characterized in that a contact point between the first resistor and the series circuit is an external terminal of the integrated circuit.