JPS6037646B2 - Feedback amplifier circuit with FET switch circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a feedback amplifier circuit equipped with an FET switch circuit which is particularly suitable for use in changing the characteristics of an equalizer circuit of a tape recorder or the like.

Generally, it is necessary for a tape recorder to switch the playback equalizer characteristics depending on the type of magnetic tape used, such as normal tape or chrome tape.

This switching has conventionally been accomplished by mechanical switches such as slide switches, but recently electronic switches using FETs (field effect transistors) as switching elements have come into use.

Compared to mechanical switches, this electronic switch can prevent poor contact and chattering, requires the shortest signal path wiring, does not cause problems such as induced noise, and has fewer circuits, making it cheaper in terms of cost. It is effective.

For this reason, FET switch circuits using FETs as switching elements have come to be used for switching the characteristics of feedback amplifier circuits such as tape recorder equalizer circuits.

That is, as shown in FIG. 1, an equalizer circuit equipped with a conventional FET switch circuit of this type is a feedback amplifier in which input and output terminals 1 and 2 are interposed and connected to a playback path (not shown) of a tape recorder. 3, and a feedback circuit 4 consisting of a plurality of resistive elements R and capacitive elements C for changing the gain or frequency characteristics of the amplifier 3 is provided between the output terminal 2 and the inverting input terminal 1' of the feedback amplifier 3. Interposed connection.

The feedback circuit 4 has a drain electrode D connected to the output terminal 2 side of the feedback amplifier 3, a source electrode S connected to the inverting input terminal 1' side, and a gate electrode connected in parallel to one resistive element R therein. G is provided with a switching field effect transistor FET connected to a movable terminal a of a changeover switch SW for switching between equalizer characteristics corresponding to normal and chrome tapes. The first fixed terminal b of the changeover switch SW is grounded, and the second fixed terminal c is connected to the negative bias power supply -V. In this way, the feedback circuit 4 is provided with the FET switch circuit 5 including the FET whose switching is controlled by the changeover switch SW. Note that the changeover switch SW sets its movable terminal a to the first
When switching to the fixed terminal b side, the FET is turned on and the equalizer characteristics of the tape recorder become normal tape characteristics, and when switching to the second fixed terminal c side, the equalizer characteristics of the tape recorder are set to the normal tape characteristics.
is turned off to set the equalizer characteristics of the tape recorder to the chrome tape characteristics.

Therefore, in a secondary equalizer circuit equipped with such a FET switch circuit, equalizer characteristics are obtained by the feedback action of the feedback circuit 4, but the O due to switching of the FET is
N, OFF control is used to obtain equalizer characteristics corresponding to normal tape characteristics and chrome tape characteristics, but in the conventional equalizer circuit described above, an input signal ei is applied to the input terminal 1 of the feedback amplifier 3. In this case, a voltage ei is similarly generated at the source electrode S of the FET, which is eventually added to the voltage VGS between the gate electrode G and source electrode S of the FET, and the input signal ei causes the channel of the FET to be The disadvantage is that the distortion at the output end of the amplifier 3 is worsened by the change in resistance.

In addition, as a means to prevent this deterioration of distortion, as shown by the dotted line in Figure 1, a low resistance Ro is inserted between the gate and source electrodes of the FET, and the gate electrode of the FET is opened using the changeover switch SW during normal tape operation. However, this method has the disadvantage that the output terminal of the amplifier 3 changes in a direct current manner by switching the changeover switch SW.

Furthermore, when using normal tape to turn on the FET,
One possible solution would be to actively open the gate electrode of the aforementioned ET to eliminate the above-mentioned drawbacks, but this would complicate the configuration and easily generate noise due to problems such as induction, so in any case, it is not practical. Not on target.

The present invention has been made in view of the above-mentioned circumstances, and has a simple and inexpensive structure that can stabilize the DC potential at the output end without causing distortion at the output end of the feedback amplifier, and can further prevent induced noise. In particular, it is an object of the present invention to provide a feedback amplifier circuit equipped with an FET switch circuit suitable for use in changing the characteristics of an equalizer circuit of a tape recorder.

Embodiments of the present invention will be described in detail below with reference to FIG. 2 of the drawings. Note that the present invention applies a feedback circuit provided in a feedback amplifier as an equalizer circuit of a tape recorder, and the same components as in FIG. I will give an explanation.

That is, the present invention is characterized by the connection configuration of the FET switch circuit 5 equipped with a switching FET provided in the feedback circuit 4 interposed and connected between the output terminal 2 and the inverting input terminal 1' of the feedback amplifier 3. It consists in composing.

That is, in the feedback circuit 4, the drain electrode D is connected to the inverting input terminal 1' of the feedback amplifier 3 in parallel to the first resistance element R on the output terminal side among the plurality of resistance elements constituting the feedback circuit 4. A switching FET is provided whose source electrode S is directly connected to the output terminal 2, and a resistance element R2 having a relatively high and low resistance is inserted between the source electrode S and gate electrode G of this FET. In addition to connecting the gate electrode G
A single switch SW for switching between normal and chrome tape characteristics.

The FET switch circuit 5 is configured by connecting to the negative bias power supply -V through the FET switch circuit 5. The functional switch SWo controls switching the equalizer characteristic of the tape recorder to the normal tape characteristic when turned on (turned on), and also turns off (opened).
It is sometimes set to control the characteristics of chrome tape. Therefore, when the cut-off switch SWo is turned on, the FE
The gate electrode G of T is negatively biased to -V and the FET is controlled to be in the ON state.On the other hand, when the changeover switch SWo is turned off, the gate electrode G is released from the negative bias of -V and the gate of the FET is controlled to be in the ON state. The electrode is supplied with a source electrode potential (output end potential) which is followed by direct current via the resistor R2, and is controlled to be in the OFF state.

Therefore, in this way, the FET is a changeover switch SW.

The equalizer characteristics are switched between normal and chrome tape characteristics by switching control by the on/off control of . Therefore, according to the circuit configuration of the present invention, when the switching of the FET is controlled by switching the changeover switch SWo, the gate potential of the FET follows the source potential due to the action of the resistor R2, and the potential between the gate and source electrodes is always a constant potential. Since the channel resistance between the gate and source electrodes is maintained at zero potential, the channel resistance between the gate and source electrodes is not subject to change due to signals, thereby preventing distortion from occurring.

Further, according to the present invention, the resistor R2 can be a large resistor since the input impedance of the FET is very large.
Therefore, since it can act as a load resistance, stable operation can be performed without affecting the output terminal of the feedback amplifier in terms of direct current.The configuration is also extremely simple, and the wiring of the signal path is easy. This has many effects such as minimizing the time and preventing the generation of induced noise.

Furthermore, the present invention is based on the above-mentioned embodiment, and as shown in FIG. 3, a pair of the basic configuration shown in FIG. 2 can be prepared for left and right channels to form a stereo stereo configuration.

That is, the gate electrodes G, .
2 are connected to each other via diodes D, D2 connected in the forward direction, and are performed by a common changeover switch SWs. By the way, the diode ○,,
The intervention of D2 is to prevent interference between both channels when the changeover switch SWs is switched.

Therefore, according to this configuration, it can be easily used as a stereo system, and furthermore, the changeover switch SWs can be used in common for both channels, so that the configuration can be further simplified by combining the effects of the above-described embodiments.

As described above, the present invention has a simple and inexpensive structure, can stably obtain a DC potential at the output end of a feedback amplifier, can prevent distortion, and can also prevent induced noise, etc., and has many advantages in practical use. It is a big thing.

[Brief explanation of the drawing]

FIG. 1 is a conventional circuit connection diagram, FIG. 2 is a circuit connection diagram showing one embodiment of the present invention, and FIG. 3 is a circuit connection diagram showing another embodiment of the present invention. 1... Input end, 2... Output end, 3... Feedback amplifier, 4... Feedback circuit, FET... Field effect transistor, SW
o...Choice switch, R2...Resistor. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1. A feedback amplifier, a feedback circuit that is interposed and connected between the input and output terminals of this amplifier and changes the gain or frequency characteristics of the amplifier, and a plurality of resistive elements that are provided in this feedback circuit and constitute the feedback circuit. A drain electrode and a source electrode are connected in parallel to the first resistive element on the output end side, corresponding to the input end side and the output end side of the amplifier, respectively, and the gate electrode is connected to a bias power source via a switch means. What is claimed is: 1. A feedback amplifier circuit equipped with a FET switch circuit, characterized in that it is comprised of a switching FET and a resistance element interposed and connected between the gate electrode and source electrode of this FET.