JPS607436B2 - IC-based low-noise preamplifier circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to IC (integrated circuit) implementation of a low-noise front layer amplifier used in a magnetic recording/reproducing device.

A typical example of a magnetic recording/reproducing device is a home video tape recorder (hereinafter referred to as a VTR), whose front layer amplifier is designed to reduce noise as explained below.
The necessary equalization characteristics are obtained by feeding back the output of each channel amplifier to the preamplifier input.

Therefore, if you try to convert such a circuit into an IC,
It becomes necessary to arrange two external terminals for the above-mentioned feedback, and this significantly limits the integration range in an IC with a limited number of pins. Figure 1: A pre-IC amplifier circuit for a follow-up VTR (portion 13 surrounded by a dotted line)
is shown in a block diagram, and the operation will be explained below.

In a VTR, frequency characteristics deteriorate as the signal passes through the magnetic tape and magnetic head system.

For this purpose, it is necessary to equip the front layer amplifier with FM reproduction equalization as shown in Figure 2, and to do this, a trimmer capacitor and a variable resistor are connected in parallel to the inductance of the magnetic head (hereinafter referred to as the head). It is generally obtained by peaking. FIG. 1 shows an example of an IJ scan two-head M type VTR amplifier.

Signals from the two heads 1 and 2 are supplied to input terminals 14 and 16 of the IC for each field, and these signals are amplified by front layer amplifiers 7 and 8.

The amplifier output is taken out from the feedback terminals 17 and 18 of the IC, and is connected to each IC input 14 and 16 by a variable resistor 5,
Negative feedback is provided through 6. Note that 9 and 10 are electronic switches that switch the respective outputs of the preamplifiers 7 and 8 for each field by a switching pulse applied from the input terminal 15.

Reference numeral 11 denotes an adder circuit that combines the outputs of the electronic switches 9 and 10.

19 is an output terminal for FM signal, 21 is an output terminal for low frequency conversion chroma signal, 22 is HPF (Takagi Pass Filter), 23
20 is a limiter input terminal, 12 is a limiter, and 24 is an FM signal output terminal.

Now, assuming that the gain of amplifiers 7 and 8 is multiplied by A, the peaking frequency in Fig. 2 is fo, the amount of peaking is Q, and the head inductance is L, the resistance value R of variable resistors 5 and 6 is approximately as follows. become. R mother 2 strokes LQ (10 A) Of course, the characteristics shown in Figure 2 are obtained by connecting variable resistors 5 and 6 in parallel to the head inductance without providing feedback terminals 17 and 18. be able to.

However, in this case, fo in the resistance R mother 2 of the variable resistors 5 and 6
This results in UQ, which becomes a large noise source, resulting in insufficient performance as a preamplifier for a VTR. Therefore, in order to achieve sufficient noise reduction, it is necessary to apply damping by feedback as shown in FIG. For this reason, feedback terminals 17 and 18 are required, and when the area around the front-layer amplifier is integrated into an IC, this extremely limits the integration range of the IC. SUMMARY OF THE INVENTION An object of the present invention is to provide a pre-IC layer amplifier for a magnetic recording/reproducing device that eliminates the above-mentioned conventional drawbacks and enables feedback damping without providing a feedback terminal on the IC. The configuration of the present invention is to amplify the output signals from a plurality of magnetic heads, switch the amplified output signals for each field period, synthesize the switched output signals, and perform low-frequency conversion of the synthesized output. In an IC-based front layer amplifier circuit of a magnetic recording and reproducing device that outputs a chroma signal or the signal and an FM signal to the outside from a low-frequency converted chroma signal output terminal,
An emitter follower circuit is inserted between the synthesized and output circuit and the output terminal for the low-frequency converted chroma signal, and the output terminal is connected to the magnetic head or the secondary side of a transformer provided at a stage subsequent to the magnetic head. This is characterized in that at least a variable resistor for negative feedback is provided between the two.

FIG. 3 shows a main part of an embodiment of the pre-IC layer amplifier circuit of the present invention.

In FIG. 3, the same parts as in FIG. 1 are given the same numbers. The difference between FIG. 3 and FIG. 1 is that the IC does not have a feedback terminal, and feedback is provided from the low frequency conversion color signal output terminal 21 to the input of the IC. To the FM signal output terminal 19 of the adder 11, an LCR circuit network as shown in FIG. 3 is connected to finely adjust the FM equalization characteristics. An open collector format is preferable for the output of the IC.

If an attempt is made to feed back from this terminal 19 to the IC input, damping will not be applied properly due to the influence of the frequency characteristics of the filter. On the other hand, since the LPF 23 connected to the low frequency conversion color signal output terminal 21 is limited to simple LPn, there is no problem in outputting it in an emitter floor format.

Therefore, even if the resistance matching LPF 23 is connected to the terminal 21, the terminal 21 is not affected by the filter at all, so that the signal at the terminal 21 can be fed back to the input section of the IC. Resistors 25, 27 and capacitor 26 are inserted for the following purposes.

In other words, without this, the gain from the input terminals 14, 16 to the terminal 21 is as large as about 3WB (generally about 1 kite Vpp input IVpp output), and the required resistance of the variable resistors 5, 6 to make Q = about 3. The value becomes too large, 50 plates, and the influence of stray capacitance becomes a problem. Therefore, resistance 2
By attenuating 5 and 27 by about 4 degrees, the resistance value of 5 and 6 which obtains the same damping characteristics can be set to about a ball circle, and the influence of stray capacitance can be ignored. Capacitor 26 is connected to terminal 14,! This is to compensate for the phase shift that occurs between the variable resistors 5 and 6 and the terminal 21, and without this, when adjusting the variable resistors 5 and 6, fo will change and adjustment will become difficult. However, this change in fo is not so large, and similar performance can be obtained without the capacitor 26.
If HPF22 is limited to a simple form, IC13
Terminals 1 and 9 are no longer needed, and terminals 2 and 2 are
1 through resistance matching.

In FIG. 3, the variable resistors 5 and 6 are directly connected to the heads 1 and 2, but in the case of a VTR, a rotary transformer is usually installed between the head and the front amplifier.
In this case, the variable resistors 5 and 6 will be connected to the secondary side of the rotary transformer.

Furthermore, in some cases, a step-up transformer is connected after the rotary transformer in consideration of noise matching with the input element of the counterfeit amplifier. But you can connect to either. As described above, according to the present invention, it is possible to reduce the number of pins of the preamplifier circuit IC by two, and
When implementing an IC around the front-layer amplifier circuit, in addition to the functions described here, it is possible to integrate a dropout compensator, etc. for large scale integration, and the cost reduction effect of IC can be greatly improved. become.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an example of a conventional VTR pre-IC layer amplifier circuit, FIG. 2 is a diagram showing FM reproduction equalization characteristics, and FIG. 3 is a block diagram showing an embodiment of the present invention. . 1, 2...Magnetic head, 7, 8...Pre-layer amplifier, 9
, 10...electronic switch, 11...adder, 13.
...IC, 21...Chroma signal output terminal, 22
...HPF, 23...LPF. 7th figure, 2nd figure, 3rd figure

Claims (1)

[Claims] 1 Amplify the output signals from a plurality of magnetic heads and switch the amplified output signals for each field period,
The output after this switching is combined and the combined output low frequency variable speed chroma signal or this signal and FM signal are outputted to the outside from the low frequency converted chroma signal output terminal. In the preamplifier circuit, at least a variable resistor for negative feedback is connected between the combining and outputting circuit and the magnetic head or a secondary side of a transformer provided after the magnetic head. IC-based low-noise preamplifier circuit.