JPS6220886Y2 - - Google Patents

Description

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

The problems when converting the head amplifier of a video tape recorder into an IC are narrowed down to two points: how to design the input impedance to be high and how to keep the amplifier noise low. However, input impedance and noise reduction are not compatible with each other, and the design is usually optimized for the inductance of the video head used. Therefore, conventional video head ICs can ensure good characteristics for heads with a specific inductance, but for heads with different inductances, the peaking characteristics described later may not be achieved, or the noise figure of the amplifier may be poor.
The problem was that it was not possible to secure a satisfactory value for NF.

The conventional problems will be explained in detail below using the drawings. Figure 1 shows a preamplifier circuit for a VHS color video tape recorder using a conventional IC. The special feature of this head amplifier is that the input signal is 0.1
It has an extremely wide band of ~7MHz, and the input level is weak at about 0.5mVpp. Therefore, the first stage amplifier needs to have good NF over the entire band from 0.1MHz to 7MHz. On the other hand, the signal source impedance is mainly determined by the inductance determined by the impedance of the video head.

The first conventional technology for improving amplifier NF is
It is common to use a peaking circuit using a trimmer capacitor 3 as shown in the figure, and the signal source resistance in this case is as shown in FIG. That is, VHS
In the case of this method, the head resonance is set at approximately 4.5 MHz (the highest frequency of the FM carrier), so the signal source resistance (Rg) reaches its maximum at the resonant frequency and decreases as it deviates from the resonant frequency. This value is, for example, a maximum value of 0.5 to 1KΩ, and a value of 5 to 10Ω below 1MHz. In the range where the signal source impedance is 1KΩ or less, the NF of the amplifier is as follows.

γ'bb...Base spread resistance of the first stage transistor (Ω) Rg...Signal source resistance (Ω) Ie...Emitter current of the first stage transistor (mA) Therefore, when trying to improve NF when Rg is small, γ' By reducing bb, the emitter current Ie
It is necessary to increase Reducing γ′bb increases the emitter-base facing length of the first-stage transistor, and as a result, the emitter-base capacitance Cbe increases. Furthermore, Ie and Cbe are in a proportional relationship, and increasing Ie also increases Cbe. In this way, when noise matching is attempted for a low signal source resistance, the input capacitance of the IC increases, creating the problem that the aforementioned head peaking cannot be performed for video heads with large inductance. . In other words, the input capacitance is too large,
This means that the resonant frequency does not reach the desired value.

For this reason, in the past, there was an inconvenience in that separate ICs had to be developed for low inductance heads and for high inductance heads.
Also, the maximum frequency of the FM carrier differs between the NTSC system and the PAL system, so the same head
The problem is that peaking characteristics cannot be achieved with the same IC.

The object of the present invention is to eliminate the above-mentioned conventional problems and to provide an integrated circuit for a video head amplifier that can be connected to video heads having a wide range of inductance values, regardless of the NTSC or PAL format.

This invention provides a dedicated power supply terminal for the first-stage amplifier, allowing the emitter current of the first-stage transistor to be controlled by the peripheral circuit constants of the integrated circuit, thereby making it possible to match the input capacitance to the inductance value of the video head used. It is something.

FIG. 3 is a circuit diagram of a main part of an embodiment in which the integrated circuit according to the present invention is used in a video tape recorder.

In FIG. 3, 1 is a video head, 2 is a rotary transformer with a winding ratio of 1:2, 3 is a trimmer capacitor for head peaking, and 4 is an integrated circuit according to the present invention. Now, we will discuss the case where the head inductance is 2 μH and it is used in an NTSC VHS video tape recorder. in this case
The maximum frequency of the FM carrier is 4.4MHz, and if the head resonance frequency is 4.5MHz, the resonance capacity is approximately
It becomes 160pF. Considering the adjustment margin and stray capacitance of the trimmer capacitor 3, the input capacitance of the integrated circuit 4 is preferably about 50 pF. Therefore, the emitter current and transistor size of the first stage transistor _Q1 are determined so that the input capacitance is approximately 50 pF.

To improve NF while keeping the input capacitance constant, increase the transistor size and reduce the emitter current. That is, the current γ′bb,Ie is γ′bb=40
Ω, Ie = 1.3mA, and if this size is doubled to make γ'bb = 20Ω and Ie = 0.65mA, the input capacitances will be approximately equal. In this case, the equivalent noise resistance Rn of the amplifier is currently Rn = 40 + 26/2 x 1.3 = 50Ω, γ'
The one with bb = 15Ω and Ie = 0.65mA is improved to Rn = 20 + 20 = 40Ω. In this way, γ′bb, Ie are optimized to satisfy the resonance condition under the lowest head resonance condition, and the head inductance is high.
For systems that require a high resonance frequency of 5.0MHz, such as the PAL system, it is sufficient to reduce Ie to satisfy the resonance conditions. In FIG. 3, transistors Q ₁ and Q ₂ constitute a cascode-type first-stage amplifier, transistors Q ₅ and Q ₆ constitute a differential-type second-stage amplifier, transistors Q ₃ and Q ₄ , and terminal 10 constitute a transistor Q _{5 .} , constitutes a DC feedback circuit to stabilize the input DC potential of _Q6 .

Terminal 6 is a first external terminal that supplies the collector current of the first stage amplifier, and terminal 7 is a second external terminal that supplies the collector current of the next stage amplifier. A current is supplied to the first external terminal 6 from a power source through a filter consisting of a resistor 9 and a capacitor 8. Therefore, if the inductance of the head used increases, the resistance value of resistor 9 will be increased and the transistor
Reduce the current flowing through Q ₁ and Q ₂ . In FIG. 3, the collectors of transistors Q ₃ , Q ₄ , and Q ₅ are connected to terminal 7, but there will be no problem if they are connected to terminal 6. However, the collector of transistor _Q6 must be connected to terminal 7. This is necessary to ensure dynamic cleansing. The DC feedback circuit in Figure 3 works to keep the collector potential of transistor _Q2 at a constant value, so the external resistor 9
Even if the DC potential of terminal 6 is lowered by increasing the value of , the collector potential of transistor Q ₂ hardly changes, and the dynamic range of the amplifier in the next stage of transistors Q ₅ and Q ₆ does not deteriorate at all.

By using the present invention, it is possible to realize an integrated circuit for a video head amplifier that can be connected to video heads having a wide range of inductance values without increasing the number of external terminals and peripheral components of the integrated circuit. That is,
The external terminal 6, capacitor 8, and resistor 9 in the embodiment of the present invention shown in FIG. 3 are originally necessary for stabilizing a high-gain amplifier, and are also used in conventional integrated circuits. . Furthermore, in the explanation of the diagram, it is written that the external terminal 7 is necessary only for the present invention, but the external terminal for the power supply voltage Vcc is naturally necessary even in conventional integrated circuits for functions other than the head amplifier. It is. As an integrated circuit for the head amplifier of a video tape recorder,
It is common to integrate not only the head amplifier but also the channel switch amplifier, AGC, dropout compensation circuit, etc. into one chip, and these functions require the power supply voltage Vcc, so the number of terminals for this does not increase. .

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing an example of a head amplifier for a video tape recorder using a conventional integrated circuit;
The figure is a graph showing the signal source resistance of the video head, and FIG. 3 is a circuit diagram showing the main parts of a head amplifier of a video tape recorder using the integrated circuit of the present invention. 6...first external terminal, 7...second external terminal, 10...DC feedback terminal.

Claims (1)

[Scope of utility model registration request] a common emitter transistor; a common base transistor whose emitter is connected to the collector of the common emitter transistor; and a load resistor connected to the collector of the common base transistor.
a filter circuit that predominantly extracts the DC component from the output signal appearing at the collector of the common base transistor; and a differential amplifier circuit that amplifies the difference signal between the output signal and the output signal of the filter circuit.
In an integrated circuit in which at least a negative feedback circuit for DC coupling an output signal appearing at the collector of the common-base transistor to the base of the common-emitter transistor is integrated on the same substrate, the collector of the common-base transistor is connected to the base of the common-base transistor through the load resistor. a first external terminal connected to a path drawn out from the terminal; and a second external terminal that supplies current to the differential amplifier circuit.
a third external terminal to which the base of the emitter-grounded transistor is connected; and a fourth external terminal to which the emitter of the emitter-grounded transistor is connected; Means for supplying collector currents of a common base transistor and a common emitter transistor are connected, the second external terminal is connected to a power supply circuit, and a means for supplying a reproduced signal from a video head is connected to the third external terminal. An integrated circuit for a video head amplifier, characterized in that the fourth external terminal is grounded.