GB2390946A

GB2390946A - Improved secondary feedback

Info

Publication number: GB2390946A
Application number: GB0215999A
Authority: GB
Inventors: Aubrey Max Sandman
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-07-10
Filing date: 2002-07-10
Publication date: 2004-01-21
Anticipated expiration: 2022-07-10
Also published as: GB2390946B; GB0215999D0

Abstract

An electronic amplifier circuit as shown in Fig. 4 comprises a first high gain amplifier unit (A1) having an inverting input terminal (10), a non-inverting input terminal (11) and an output terminal (12). An input signal is supplied from a circuit input terminal (T1) through a series input resistor (R1) to the inverting input terminal (10) of the first high gain amplifier unit (A1). A negative feedback resistor (R2) is connected between the output terminal (12) and the inverting input terminal (10) of the first high gain amplifier unit (A1). An auxiliary high gain amplifier unit (A4), preferably having a gain of at least 100, has its inverting input terminal (30) connected to the junction 23 between the resistors R1 and R2, and its output terminal (32) connected to the non-inverting input terminal (11) of the first high gain amplifier unit (A1). The non-inverting input terminal 31 of A4 is connected to earth.

Description

1ECTRONIC AMPLIFIER CIRCUIT 2390946

Technical Field

This invention relates to electronic amplifier circuits. It has application in electronic amplifier circuits that utilise high gain amplifier units such as operational amplifiers, and other amplifiers using either discrete transistor circuits or integrated circuits.

Background Art

In general operational amplifiers have two input terminals, namely an inverting input terminal and a non-invertin input terminal, and an output terminal. If an input signal is applied to the inverting terminal and the non-inverting input terminal is connected to ground then a highly amplified version of the input signal appears at the output terminal and the output signal is in anti-phase to the input signal.

In practical operational amplifiers, although the gain is very high it is frequency sensitive and typically above a certain frequency reduces at around 20dB per decade. To improve the constancy of the gain of an operational amplifier it is known to provide negative feedback. Such negative feedback is provided by a resistive path from the output terminal of the operational amplifier to its inverting input terminal (Fig. I). Negative feedback increases the frequency range over which the gain is constant, albeit with a reduction of the gain compared with its open loop value.

However a further problem with practical operational amplifiers is the generation of harmonic distortion within the amplifier itself. This distortion arises due to internal imperfections in the components and is still present even when negative feedback is provided and in fact increases with signal frequency. Typical values for harmonic distortion are 0.001% at 1kHz, 0.01% at IOkHz and 0. 1% at 100klIz. The distortion increases by around 20dB per decade as the frequency increases.

Where an operational amplifier is being used as an audio signal amplifier a figure of even 0.01% distortion at 10kHz can give rise to audible interrnodulation distortion signals when multiple audio input signals of different but similar frequencies are applied to such an amplifier. In certain radio frequency applications such as transmitter circuits for cellular telephones such distortion can make an operational amplifier unacceptable as an amplifier circuit Additionally certain electronic components such as active filters and oscillators require low levels of harmonic distortion to operate effectively. Similar effects are encountered with other types of amplifiers that also require some means for reducing the distortion produced in the amplifier itself. One suggested solution to the problem of distortion is described in US Patent No. 3,825,854 to Pichal. In the circuits described in that patent specification, a proportion of the difference

between the input and output signals, obtained from the junction between two resistors connected in series between the input and output terminals, is fed back to the non-inverting input of the operational amplifier through a phase inverter having a gain of either -1 or -2. Practical embodiments of the circuits described in Pichal have been shown to be unsatisfactory in reducing distortion (Fig.2).

A second approach to reducing harmonic distortion is suggested in the patent application No. XX by Sandman, using the principle of secondary feedback in which an auxiliary amplifier injects a signal, comprising essentially only the harmonic distortion products, back into the main amplifier in order to cancel the original distortion to a large degree. The Sandman secondary feedback circuit has shown a much greater reduction in distortion than is achievable with the Pichal circuit (Fig.3).

It is an object of the present invention to provide a circuit that reduces the distortion at least as electives as the secondary feedback circuit but with a much simpler design and reduced component count.

Disclosure of the Invention

According to the invention an electronic amplifier circuit comprises: an input terminal and an output terminal, a first high gain amplifier unit having two types of input terminal, namely an inverting input terminal and a non-inverting input terminal, and an output terminal, which output terminal is connected to the output terminal of the circuit, a connection including series resistance from the input terminal of the circuit to the inverting input terminal of the first amplifier unit, a negative feedback resistive path from the output terminal of the first amplifier unit to, its inverting input terminal, at an intermediate connection point, an auxiliary high gain amplifier unit preferably having an open-loop gain of at least 100 times, and having two types of input terminal, namely an inverting input terminal and a non-inverting input terminals and an output terminal, said intermediate connection point being connected to the inverting input terminal of the auxiliary nigh gain amplifier unit and the output terminal of the auxiliary high gain amplifier unit being connected to the non-inverting input terminal of the first high gain amplifier unit. The non-inverting input terminal of the auxiliary amplifier is connected to earth (Fig.4). In carrying out the invention the output terminal of the auxiliary amplifier may be connected to the non-inverting terminal of the main amplifier through a series resistor in which case resistance and capacitance in series can be connected between the non-inverting input terminal of the first amplifier unit and ground.

In order that the invention may be more t-ully understood reference will now be made to the accompanying drawings in which: Fig. I is a circuit diagram of an electronic amplifier circuit known from the prior art,

Fig. 2 is a circuit diagram of an electronic amplifier circuit known from the Pichal patent specification,

Fig 3 is a circuit diagram of an electronic amplifier circuit known from the Sandman patent application No. XX c:/c 55 ILL) Fig. 4 is a circuit diagram of an electronic amplifier circuit embodying the invention, and Fig.5 is a circuit diagram of another embodiment of the invention.

ferring now to Fig. 1 there is shown therein, from the prior art, a circuit diagram of an

electronic amplifier circuit having an input terminal Tl and an output terminal T2.

Mnplification is provided by a high gain main amplifier unit A1. In this embodiment unit A1 comprises an operational amplifier having two types of input terminal, namely an inverting input terminal 10 and a non-inverting input terminal 1 l, and an output terminal 12. A series resistor Al is connected between circuit input terminal T1 and inverting input terminal 10 of amplifier unit A1. Output terminal 12 of unit Al is connected to the output terminal T2 of the circuit. A negative feedback resistor R2 is connected between output terminal I and inverting input tcrrninal 10 of amplifier unit A1. Non-inverting input terminal I I of amplifier unit A1 is connected to ground.

For the above circuit if a feedback factor is den ned as R1;(RI R2) then the overall amplification G of the circuit t or an open loop gain A of amplifier unit A I provided with negative feedback is given by G = A,'[1 + A*RI/(RI By)]. ( 1) The value of the open loop gain A is frequency dependent but the effect of the negative feedback is to maintain G constant over a wide range of frequencies, albeit at the cost of scone reduction of the gain below its Open loop value A However, frequency dependent distortion is introduced in practical operational amplifier units and is present in sufficient magnitude to be a problem even with the inclusion of negative feedback.

It can be shown that the reduction factor F I for harmonic distortion for the Fig. 1 circuit having an amplifier unit A1 with an open loop gain A and provided with negative feedback is given by F1 = 1/ [1 Rl*l.\l l (R1 + R2)] (2) where A is the magnitude of the open loop gain A of amplifier unit A I While such a circuit does give an improvement in distortion in comparison with an open loop operational amplifier unit that is not provided with negative feedback, the problem of distortion still persists.

In an endeavour to mitigate the effects of distortion, the circuit of Fig. 2 has been proposed in US Patent No. 3,825,854 to Pichal. In Fig.2 like parts have like reference numerals to Fig. 1.

Additional components in the c ircuit of Fig. 2 comprise a pair of resistors R3 and R4 connected ire series between input terminal T1 and output terminal T2. A second amplifier unit A2 is p rovided having an inverting input terminal 20 and an output terminal 22. A connection is taken from junction 23 between resistors R3 and R4 to inverting terminal 20 of amplifier A2 and the output terminal 22 of unit A2 is connected to the non-inverting terminal 11of amplifier unit A1.

Unit A2 provides inversion only and no amplification. In practice A2 consists of a voltage follower and an inverter. Its gain is therefore -; (or in one embodiment -2).

It can be shown that the harmonic distortion F2 for the Fig. 2 circuit with an open loop gain A for amplifier unit Al is given by Fat = 1 / [l + {Rl /(RI R2) + R3i(R3 R4)3.Aj] (3) where IAI is the magnitude of A

itch a circuit gives some further improvement in distortion, in comparison with the negative feedback circuit of Fig. 1, but the improvement is still not very significant, of the order of 6dB.

The circuit described in the Sandman patent which uses the method of secondary feedback and which results in a dramatic reduction of distortion is shown in Fig. 3. in Fig. 3 like components have like reference numerals to Fig. I and Fig. 2. However, in Fig.3 a high gain open loop amplifier unit A3 is provided instead of the unity gain inverting amplify er unit A2 that is shown in Fig. 2. Amplifier unit A3 has an inverting input terminal 30, a non-inverting input terminal 31 and an output terminal 32. A connection is made from junction 23 between resistors R3 and Ret to inverting input terminal 30. Non-inverting terminal 31 is grounded and the output terminal is connected through an optional series resistor R5 to inverting input terminal 10 of amplifier unit Al. Amplifier unit A3 provides auxiliary feedback to the input of main amplifier unit Al. This auxiliary feedback represents a highly amplified version of a proportion of the difference between the input signal at Tl and the distorted output signal at To. This amplified signal acts to significantly reduce the distortion. It can be shown that the reduction factor F3 for the circuit of Fig. 3 is given by F3 = I, [I + !AI/(l+R2iR1+R2,R5) -!AI:B!*R2*R3lR5(l-R2//Rl-R2fR5)(R3+R4)] (4) where BI is the magnitude of the open loop gain B of amplifier unit A3. Preferably the value of open loop gain B is very much greater than the ratio R2/R1. Typically R2/1 = 10 and B 100.

The substantial further reduction in harmonic distortion over that obtained with either conventional negative feedback as shown in equation (2), or with the Pichal circuit as shown in equation (3), is therefore shown by equation (4) to be the result of the open loop gain B of the auxiliary amplifier unit A3 This gain should therefore be as large as possible consistent with maintai ning stable operation.

Additional circuits described in the Sandman patent application No. X also produce significant reduction in distortion, but all of these circuits require virtually exact balance between the ratios R1/R) and R3/R4, a condition difficult to satisfy in practical circuits.

In the invention described in this patent, the practical disadvantages of secondary feedback as employed in the Sandman patent application No.XX are overcome by the adoption of a much simpler design using fewer components, as shown in Fig. 4. The input to the auxiliary amplifier A4 is taken from the intermediate connection point 23 and connected to the inverting terminal 30 of A4. The non-inverting terminal 31 of A4 is connected to earth. The output terminal 32 of A] is connected to the non- inverting input terminal I I of the main amplifier A I. The reduction factor F4 for harmonic distortion for this circuit is given by: F4 = I! [ 1 + R I *IAI( 1!Bj)/(R I +R2)] (5) where BI is the magnitude of the open loop gain B of the auxiliary amplifier A4. The elimination of the resistances R3 and R4, used in the secondary feedback circuits shown in Fig. 3, simplify es the design considerably and also substantially reduces the sensitivity of the circuit to variations in component values.

Claims

1. An electronic amplifier circuit comprising: an input terminal and an output terminal, a first high gain amplifier unit having To types of input terminal, namely an inverting input terminal and a non-inverting irput terminal, and an output terminal, which output terminal is connected to the output terminal of the circuit.

a connection including series resistance from the input terminal of the circuit to the inverting input terminal of the first amplifier unit, a negative feedback resistive path from the output terminal of the first amplifier unit to its .. mvertmg input terminal, a resistive path provided between the circuit output terminal and the circuit input terminal, which resistive path has an intermediate connection point.

an aL,ciliary high gain amplifier unit having two types of input terminal namely an inverting input terminal and a non-inverting input terminal, and an output terminal, said intermediate connection point being connected to the inverting input terminal of the auxiliary high gain amplifier Unit and the output terminal of the auxiliary high gain amplifier unit being connected to the non-inverting input terminal of the first high gain amplifier unit.

2. The circuit as claimed in claim 1 in which the gain of the auxiliary high gain amplifier is preferably at least 100.

3. The circuit as claimed in either one of the preceding claims in which said intermediate connection point is connected to the inverting input terminal of the auxiliary high gain amplifier and the output terminal of the auxiliary amplifier is connected to the non-inverting terminal of the main amplifier.

4. The circuit as claimed in any one of the preceding claims in which a series resistance is included between the output terminal of the auxiliary amplif er and the said input terminal of the first amplifier unit which is connected to earth via a resistance-capacitance combination.

g. 5 shoves a farther embodiment of the invention in which a compensating circuit comprising a series resistance and a series resstance-capacitance combination provides a farther improvement in distortion reduction.