GB1565974A

GB1565974A - Recording amplifiers for magnetic tape recorders

Info

Publication number: GB1565974A
Application number: GB3882876A
Authority: GB
Original assignee: Ricoh Co Ltd
Current assignee: Ricoh Co Ltd
Priority date: 1975-09-22
Filing date: 1976-09-20
Publication date: 1980-04-23
Also published as: JPS5238212A

Description

(54) IMPROVEMENTS IN AND RELATING TO RECORDING AMPLIFIERS FOR MAGNETIC TAPE RECORDERS (71) We, RICOH COMPANY, LTD., a Japanese body corporate of 34 l-chome Naka Magome, Ohta-ku, Tokyo, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to recording amplifiers for magnetic tape recorders.

According to the invention there is provided a recording amplifier for a magnetic tape recorder comprising an input stage having a pair of input terminals for connection to a microphone, an output stage having a pair of output terminals for connection to a magnetic recording head, each said stage comprising a respective transistor, and constant current means connected in series with the main current path of the transistor of the output stage and operable to maintain the magnitude of the current through the said main current path substantially constant irrespective of the variations in voltage across the output terminals.

According to the invention, there is further provided a recording amplifier for a magnetic tape recorder comprising an input stage having a pair of input terminals for connection to a microphone, an output stage having a pair of output terminals for connection to a magnetic recording head, the input stage including a field effect transistor, the output stage including a main transistor and control means connected in series with the main current path of-the main transistor to maintain the magnitude of current through the main transistor substantially constant irrespective of the voltage variations fed by the main transistor to the output terminals, and feed-back means responsive to the signal at the junction between the main transistor and the control means to vary the bias voltages applied both to the field effect transistor and the main transistor in a sense to maintain the operating points of the two transistors substantially constant irrespective of variations in temperature and supply voltage.

Amplifiers for driving magnetic recording heads and embodying the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is an electrical circuit diagram of magnetic tape recording apparatus incorporating a said amplifier; and Figure 2 is an electrical circuit diagram of another magnetic tape recording apparatus incorporating another said amplifier.

The magnetic recording apparatus 41 shown in Figure 1 has an electret condenser microphone element 42 which is connected in series with a resistor 43 and a capacitor 45 between a DC voltage source +V and ground. The junction between the microphone element42 and the resistor 43 is connected to the gate of an N-channel junction field-effect (JFET) transistor 44.

The JFET transistor 44 is connected in a source follower arrangement with the drain thereof connected to the voltage source +V and the source thereof connected through a source resistor 46 to the capacitor 45, the resistor 46 constituting a load resistor for the source circuit of the JFET transistor 44 and also providing self-bias. The microphone element 42, resistors 43 and 46 and JFET transistor 44 are housed in a casing designated as 47.

The source of the JFET transistor 44 is directly coupled to the base of a PNP bipolar transistor 48 which is connected in a common-emitter arrangement. The emitter of the bipolar transistor 48 is connected to the voltage source +V through a resistor 49.

The collector of the bipolar transistor 48 is connected to the collector of an NPN bipolar transistor 51, the emitter of which is grounded. The base of the bipolar transistor 51 is connected to ground through a capacitor 52 and to the collector of the bipolar transistor 51 through a resistor 53.

The collector of the bipolar transistor 48 is also connected to a magnetic recording head 54 through a coupling capacitor 56 and a recording bias circuit 57 is connected to the recording head 54 in a conventional manner.

The transistors 44 and 48 constitute input and output stages of a recording amplifier (not designated) of the apparatus 41 and the transistor 51 constitutes a collector circuit load for the transistor 48. The resistor 53 provides self-bias for the transistor 51 through degenerative feedback. The capacitor 52 is selected to have negligible reactance at all operating frequencies of the apparatus 41 and places the base of the transistor 51 at AC ground. The base current and therefore the collector current of the transistor 51 are maintained at a predetermined value by the action of the feedback resistor 53 and the transistor 51 thereby constitutes a constant current source. Since substantially the same collector current must flow through both transistors 48 and 51, when both are in a conductive state (the resistance of the resistors 58 and 59 being relatively high), the collector current of the transistor 48 is maintained substantially constant by the transistor 51.

The collector of the transistor 48, which constitutes the output of the recording amplifier, is grounded through the series combination of resistors 58 and 59 which constitute a DC feedback voltage divider.

The junction of the resistors 58 and 59 is connected to the junction of the source resistor 46 and the capacitor 45. The capacitor 45 has negligible reactance at all operating frequencies of the apparatus 41 and maintians the junction of the resistors 58 and 59 at AC ground. The resistors 58 and 59 and the capacitor 45 constitute a DC negative feedback loop which will be described in detail below.

In operation, the signals from the microphone element 42 are amplified by the JFET transistor 44 and directly coupled to the bipolar transistor 48 which further amplifies the signals and applies them to the recording head 54. Although the voltage at the collector of the transistor 48 varies in accordance with the signals from the microphone element 42, there is no change in collector current regardless of variations in signal voltage and signal frequency. The frequency response of the recording head 54 to the output of the recording amplifier will therefore be substantially linear; that is the amplitude of the recording signal current through the head is substantially constant with variations in signal frequency.

The regulated current through the transistor 51 also provides a means of biasing both transistors 44 and 48 and maintaining the operating points thereof at predetermined values regardless of temperature and power source variations.

In the configuration shown, the source resistor 46 serves to self-bias both transistors 44 and 48 and maintain the operating points thereof at predetermined values by degenerative feedback. Through proper selection of the values of the resistors 53,46, 58 and 59, the DC operating points of both the transistors 44 and 48 may be adjusted to optimum values, and these operating points are maintained through the regulating action of the transistor 51. The problem of adjusting the operating points of the transistors 44 and 48 to values which would be suitable for both transistors is overcome in the present apparatus 41 since the voltage at the lower end of the resistor 46 may be adjusted to any required value by suitable selection of the voltage dividing resistors 58 and 59.

Figure 3 shows another embodiment of the present invention which is essentially similar to the embodiment of Figure 2. In the magnetic tape recording apparatus 61 shown in Figure 2 parts similar to those in Figure 1 are similarly referenced. In this apparatus 61 the base of the transistor 48, rather than being connected directly to the source of the transistor 44, is connected thereto through transistors 62 and 63. The transistors 62, 63 and 48 are connected in cascade in a complimentary symmetry amplifier configuration with the transistors 62 and 63 providing two additional gain stages. The transistor 62 is of the PNP bipolar type and its base is directly connected to the source of the transistor 44.

The collector of the transistor 62 is grounded through a resistor 64 and the emitter thereof is connected to the voltages source +V through an emitter resistor 66.

The transistor 63 is an NPN bipolar type and has its base directly connected to the collector of the transistor 62. The emitter of the transistor 63 is grounded and the collector thereof is connected to the voltage source +V through a resistor 67 and also directly to the base of the transistor 48. An automatic gain control (AGC) circuit 68 of conventional construction is connected between the collector of the transistor 63 and the emitter of the transistor 62. The signal level at the collector of the transistor 63 is sensed by the AGC circuit 68, which accordingly varies the emitter voltage of the transistor 62 developed across the resistor 66 to maintain the signal level applied to the recording head 54 at a constant selected level. The operation of the apparatus 61 is generally similar to that of the apparatus 41 with the exception that two more gain stages and AGC are provided.

Although the JFET transistor 44 illustrated is an N-channel type and is connected in a source-follower arrangement it can instead be replaced by a P-channel type with the source and drain connections reversed and the value of the resistor 46 suitably changed. A P-channel field-effect transistor connected in this manner would constitute a common source amplifier providing higher voltage gain.

WHAT WE CLAIM IS: 1. A recording amplifier for a magnetic tape recorder comprising an input stage having a pair of input terminals for connection to a microphone, an output stage having a pair of output terminals for connection to a magnetic recording head, each said stage comprising a respective transistor, and constant current means connected in series with the main current path of the transistor of the output stage and operable to maintain the magnitude of the current through the said main current path substantially constant irrespective of the variations in voltage across the output terminals.

2. An amplifier according to claim 1 wherein the output stage transistor comprises a bipolar transistor.

3. An amplifier according to claim I or to claim 2 wherein the input stage transistor comprises a field-effect transistor.

4. An amplifier according to claim 3, as dependent upon claim 2, wherein the source of the field effect transistor is directly connected to a base of the bipolar transistor.

5. An amplifier according to claim 3 as dependent upon claim 2, wherein the drain of the field-effect transistor is directly connected to the base of the bipolar transistor.

6. An amplifier according to claim 4, including DC feedback circuit means connected between an output of the output stage and an input of each of the input and output stages.

7. An amplifier according to claim 6, wherein the input stage further comprises a source circuit resistor, the feedback circuit means being connected to the source circuit of the field-effect transistor and to the base of the bipolar transistor through the source circuit resistor.

8. An amplifier according to claim 7, wherein the field-effect transistor is connected in a common drain configuration.

9. An amplifier according to claim 6, wherein the feedback circuit means comprises a voltage divider.

10. An amplifier according to any preceding claim, as dependent upon claim 2, wherein the constant current means is connected as a collector load of the bipolar transistor.

11. A recording amplifier for a magnetic tape recorder comprising an input stage having a pair of input terminals for connection to a microphone, an output stage having a pair of output terminals for connection to a magnetic recording head, the input stage including a field effect transistor, the output stage including a main transistor and control means connected in series with the main current path of the main transistor to maintain the magnitude of current through the main transistor substantially constant irrespective of the voltage variations fed by the main transistor to the output terminals, and feedback means responsive to the signal at the junction between the main transistor and the control means to vary the bias voltages applied both to the field effect transistor and the main transistor in a sense to maintain the operating points of the two transistors substantially constant irrespective of variations in temperature and supply voltage.

12. An amplifier according to claim 11 wherein the input and output stages are DC coupled.

13. An amplifier substantially as herein described with reference to Figure 1 of the accompanying drawings.

14. An amplifier substantially as herein described with reference to Figure 2 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. connected in a source-follower arrangement it can instead be replaced by a P-channel type with the source and drain connections reversed and the value of the resistor 46 suitably changed. A P-channel field-effect transistor connected in this manner would constitute a common source amplifier providing higher voltage gain. WHAT WE CLAIM IS:

1. A recording amplifier for a magnetic tape recorder comprising an input stage having a pair of input terminals for connection to a microphone, an output stage having a pair of output terminals for connection to a magnetic recording head, each said stage comprising a respective transistor, and constant current means connected in series with the main current path of the transistor of the output stage and operable to maintain the magnitude of the current through the said main current path substantially constant irrespective of the variations in voltage across the output terminals.