Single ended differential amplifier bjt
The operational amplifier is a direct-coupled high gain amplifier usable from 0 to over 1MH Z to which feedback is added to control its overall response characteristic i. The op-amp exhibits the gain down to zero frequency. Such direct coupled dc amplifiers do not use blocking coupling and by pass capacitors since these would reduce the amplification to zero at zero frequency. Large by pass capacitors may be used but it is not possible to fabricate large capacitors on a IC chip.
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Basic Electronics | BJT Amplifiers
Year of fee payment : 4. Year of fee payment : 8. Year of fee payment : A differential to single-ended signal transfer circuit that allows increased gain and improved AC performance while reducing power supply voltage requirements.
The transfer circuit includes a first operational transconductance amplifier OTA , a second operational amplifier OPA , first and second controlled current sources, a third current source, and first and second bipolar junction transistors. The inverting and non-inverting inputs of the transfer circuit are provided at the inverting input and the non-inverting input, respectively, of the OTA, which is coupled to the first and second controlled current sources to form a current mirror with tracking feedback.
The output voltage of the transfer circuit is provided at the emitter of the first transistor, the base of which is connected to the non-inverting input INp.
The first transistor is coupled to the third current source in an emitter follower configuration to provide both current gain and impedance matching.
The base of the second transistor is connected to the inverting input of the transfer circuit. The OPA is coupled between the respective emitters of the first and second transistors in a voltage follower configuration.
The present application relates generally to improvements in differential to single-ended signal transfer circuits, and more specifically to techniques for increasing the open loop gain, for improving the AC performance, and for reducing the power supply voltage requirements of operational amplifiers including such transfer circuits.
Two-stage bipolar operational amplifiers are known that include a differential input stage and a single-ended second stage. For example, a conventional two-stage bipolar operational amplifier op amp may include a differential input stage, a differential to single-ended signal transfer circuit, a single-ended second stage, and an output buffer.
In the conventional two-stage bipolar op amp, the differential to single-ended transfer circuit may comprise a current mirror, the single-ended second stage may comprise a common emitter circuit, and the output buffer may comprise a voltage follower circuit.
Such a two-stage bipolar op amp typically has two high impedance nodes—one at the output of the first stage, and another at the output of the second stage.
The conductance associated with these high impedance nodes, which is related to the transconductance of the first and second stages, determines the gain of the respective stages. For example, the gain of each stage may range from about to about , and the open loop gain of the two-stage bipolar op amp may range from about , to about 1,, To increase the operating voltage range of the conventional two-stage bipolar op amp, the output buffer may be omitted, and the output of the op amp may be provided at the second stage output.
In this way, a two-stage bipolar op amp may be configured having a wide operating voltage range that approaches a rail-to-rail voltage swing. In this configuration, however, the load resistance is applied directly to the second stage output, which can reduce the gain of the second stage to about 10— based on the quiescent current level and the load resistance. One way of increasing the open loop gain of a two-stage bipolar rail-to-rail op amp is to reduce the conductance at the output of the first stage i.
To this end, the differential to single-ended transfer circuit comprising the current mirror may be provided with tracking feedback. For example, FIG. The transfer circuit further includes a transistor Q 3 configured to provide tracking feedback for the current mirror , and a current source I 0 connected to the respective emitters of the transistors Q 1 —Q 3.
As shown in FIG. Further, the non-inverting input INp of the transfer circuit a , which corresponds to the base connection of the transistor Q 3 see FIG. Like the transfer circuit , the output of the transfer circuit a is provided at the base connection The transfer circuit a includes two current sources I 1 and I 2 , both of which are controlled by the feedback amplifier In general, providing a current mirror with tracking feedback, as depicted in FIGS.
Because the input and output voltages of the current mirror are equal to one another, the input and output currents of the current mirror e. As a result, the open loop gain of the two-stage bipolar op amp incorporating the transfer circuit or a see FIGS.
One drawback of the transfer circuits and a employing current mirrors with tracking feedback is that the input conductance associated with the single-ended second stage can cause the differential input stage to become imbalanced, thereby degrading the open loop gain of the two-stage bipolar op amp. Current gain can be increased by increasing the number of buffers between the input and the output of the second stage.
However, because this approach increases the number of sub-stages within the second stage, the AC performance and the stability of the op amp may be adversely affected.
Another way of increasing the open loop gain of the two-stage bipolar rail-to-rail op amp is to employ a base current cancellation technique. The transfer circuit includes transistors Q 1 —Q 2 , controlled current sources I 1 , and I 2 , and a current source I 3. However, the base current cancellation technique of FIG. For example, due to the different collector-emitter voltages of the transistors Q 1 —Q 2 of the transfer circuit , the betas of the transistors Q 1 —Q 2 may be different, thereby causing an error in the base current cancellation.
Further, the transfer circuit is normally not suited for use with low power supply voltages due to the additional voltage drop across the transistor Q 2. Moreover, the inverting and non-inverting inputs INn and INp of the transfer circuit may be imbalanced, thereby exacerbating the base current cancellation error.
In addition, in the event a boosted stage configuration of the transfer circuit is employed see, e. It would therefore be desirable to have a two-stage bipolar rail-to-rail op amp including a differential to single-ended transfer circuit that avoids the drawbacks of conventional op amp circuit configurations.
In accordance with the present invention, a two-stage bipolar rail-to-rail operational amplifier op amp is provided including a differential to single-ended signal transfer circuit that allows increased open loop gain and improved AC performance, while reducing operating power supply voltage requirements. The differential to single-ended transfer circuit provides highly accurate base current cancellation without lengthening the signal path within the circuit.
In one embodiment, the two-stage bipolar rail-to-rail op amp includes a differential input stage, the differential to single-ended signal transfer circuit, a single-ended second stage, and an output buffer. The differential to single-ended transfer circuit includes a first operational transconductance amplifier OTA , a second operational amplifier OPA , first and second controlled current sources, a third current source, and first and second bipolar junction transistors.
The inverting and non-inverting inputs INn and INp of the transfer circuit are provided at the inverting input and the non-inverting input, respectively, of the OTA, which is coupled to the first and second controlled current sources to form a current mirror with tracking feedback. The OTA provides sufficient feedback to assure that the voltage difference between the input of the current mirror i.
The output voltage Vout of the transfer circuit is provided at the emitter of the first transistor, the base of which is connected to the non-inverting input INp. The low output impedance of the emitter follower is used to match a low impedance load, such as the input of the single-ended second stage of the two-stage op amp. The base of the second transistor is connected to the inverting input INn. Because the voltage difference between the inverting input INn and the non-inverting input INp is low, the base voltage of the second transistor is substantially equal to that of the first transistor.
Further, the OPA is coupled between the respective emitters of the first and second transistors in a voltage follower configuration to assure that the emitter voltage of the second transistor is substantially equal to that of the first transistor.
As a result, the operating points of the first and second transistors are substantially the same. Further, the respective base currents of the first and second transistors substantially cancel one another, thereby allowing the input current of the transfer circuit to be reduced while increasing the input impedance of the circuit. Such base current cancellation is obtained without increasing the length of the signal path within the transfer circuit. Further, the increased input impedance of the transfer circuit is achieved while reducing the parasitic conductance associated with the high impedance node.
By providing high input impedance with reduced parasitic conductance, and by implementing base current cancellation off of the signal path, the presently disclosed differential to single-ended transfer circuit may be incorporated into a two-stage bipolar rail-to-rail op amp to obtain increased open loop gain and improved AC performance.
Other features, functions, and aspects of the invention will be evident from the Detailed Description of the Invention that follows. The invention will be more fully understood with reference to the following Detailed Description of the Invention in conjunction with the drawings of which:.
A differential to single-ended signal transfer circuit is disclosed that may be employed in a two-stage bipolar rail-to-rail operational amplifier op amp. The presently disclosed differential to single-ended signal transfer circuit allows the two-stage bipolar rail-to-rail op amp to have increased open loop gain, improved AC performance, and reduced operating power supply voltage requirements. In the illustrated embodiment, the transfer circuit comprises a current mirror , a base current cancellation circuit , and an output buffer Specifically, the current mirror includes an operational transconductance amplifier OTA , and first and second controlled current sources I 1 and I 2.
The base current cancellation circuit includes a second operational amplifier OPA configured as a voltage follower, and an emitter-follower transistor Q 2. The output buffer includes an emitter-follower transistor Q 1 , and a third current source I 3.
In the presently disclosed embodiment, the transfer circuit is implemented on a bipolar integrated circuit IC. It is understood, however, that the various components of the transfer circuit may be implemented using any suitable IC technology.
Further, the current mirror has an input at the terminal INn and an output at the terminal INp, which correspond to the inverting and non-inverting inputs of the OTA , respectively. The current sources I 1 and I 2 are controlled by the output of the OTA , which operates as a feedback amplifier to provide the current mirror with tracking feedback. In the presently disclosed embodiment, the OTA and the controlled current sources I 1 and I 2 are configured such that the voltage level at the current mirror input INn is substantially equal to the voltage level at the current mirror output INp.
As a result, the input and output currents of the current mirror are substantially equal to one another for any given output voltage Vout, thereby making the differential inputs INn and INp of the transfer circuit well balanced.
As described above, the output buffer includes the emitter-follower transistor Q 1 and the current source I 3. Those of ordinary skill in this art will appreciate that such an emitter follower circuit configuration provides both current gain and impedance matching. For example, the emitter current of the emitter-follower transistor Q 1 is typically about times the base current of the transistor Q 1. Accordingly, the input impedance of the output buffer is significantly greater than the output impedance of the buffer , thereby allowing the output Vout of the transfer circuit to match a low impedance load.
Moreover, the base current cancellation circuit includes the OPA and the emitter-follower transistor Q 2. As described above, the OPA is coupled between the respective emitters of the transistors Q 1 —Q 2 as a voltage follower, thereby assuring that the emitter voltage of the transistor Q 2 is substantially equal to the emitter voltage of the transistor Q 1.
Further, because the voltage level at the input INn is substantially equal to the voltage level at the input INp, the base voltages of the transistors Q 1 —Q 2 are substantially equal to one another. As a result, the operating points of the emitter-follower transistors Q 1 —Q 2 are substantially the same, and the base currents of the transistors Q 1 —Q 2 are substantially equal and therefore cancel one another.
In this way, the input impedance of the transfer circuit is increased. Like the transfer circuit , the transfer circuit comprises the current mirror , the base current cancellation circuit , and the output buffer The transfer circuit further includes transistors Q 3 and Q 4 , and a fourth current source I 4. Specifically, the transistor Q 3 and the current source I 4 are configured to boost the current gain of the output buffer including the transistor Q 1.
Further, because the addition of the transistor Q 3 and the current source I 4 may change the collector voltage of the transistor Q 1 , the diode-connected transistor Q 4 is added to the transfer circuit and configured to assure that the collector voltage of the transistor Q 2 is substantially equal to that of the transistor Q 1.
Like the transfer circuit , the transfer circuit comprises the current mirror , the base current cancellation circuit , the output buffer , the transistor Q 3 , and the current source I 4.
However, in place of the diode-connected transistor Q 4 included in the transfer circuit , the transfer circuit includes a third operational amplifier OPA coupled between the respective collectors of the transistors Q 1 —Q 2 as a voltage follower, thereby assuring that the collector voltage of the transistor Q 2 is substantially equal to the collector voltage of the transistor Q 1. Specifically, the OTA comprises a differential stage and a current mirror Further, the current mirror includes a diode-connected input transistor Q 8 and an output transistor Q 7 coupled to the transistor Q 8 , and emitter degeneration resistors R 3 and R 4 coupled between the transistors Q 8 and Q 7 and the power supply voltage Vcc, respectively.
The OPA comprises a differential stage and a current mirror Further, the current mirror includes a diode-connected input transistor Q 14 and an output transistor Q 13 coupled to the transistor Q 14 , and emitter degeneration resistors R 5 and R 6 coupled between the transistors Q 13 and Q 14 and the power supply voltage Vee, respectively.
The OPA further includes an output transistor Q 15 , and an emitter degeneration resistor R 7 coupled between the transistor Q 15 and the supply voltage Vee. Similarly, the current source I 2 includes an output transistor Q 6 , and an emitter degeneration resistor R 2 coupled between the transistor Q 6 and the supply voltage Vcc. It is noted that capacitors C 1 , C 2 , and C 3 and a resistor R 8 are included in the transfer circuit to provide frequency compensation.
Like the transfer circuit , the transfer circuit comprises the OTA , the OPA , and the output buffer The transfer circuit further comprises a differential input stage , a folded cascode section , and a class-A output stage coupled to the output buffer The folded cascode section includes NPN-type BJTs Q and Q biased by a voltage source Vbias, and emitter degeneration resistors R 1 —R 2 coupled between the transistors — and the supply voltage Vee, respectively.
The class-A output stage includes an emitter-follower transistor Q , and a current source I The transfer circuit further includes a first AB-control circuit , a second AB-control circuit , an output stage , and a reference voltage source REF
1.6: The Differential Amplifier
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Most modern operational amplifiers utilize a differential amplifier front end. In other words, the first stage of the operational amplifier is a differential amplifier. This circuit is commonly referred to as a diff amp or as a long-tailed pair. A diff amp utilizes a minimum of 2 active devices, although 4 or more may be used in more complex designs. Our purpose here is to examine the basics of the diff amp so that we can understand how it relates to the larger operational amplifier. Therefore, we will not be investigating the more esoteric designs. To approach this in an orderly fashion, we will examine the DC analysis first, and then follow with the AC small signal analysis. Note the inherent symmetry of the circuit. If you were to slice the circuit in half vertically, all of the components on the left half would have a corresponding component on the right half.
Bjt differential amplifier examples
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The circuit diagram of a differential amplifier using one opamp is shown below. BJT Differential Amplifier using active loads: A simple active load circuit for a differential amplifier is the current mirror active load as shown in figure. How the transistor Q2 also producing output voltage even though the input is provided only to transistor Q1? The term microprocessor and microcontroller can be confusing for those who are new to this field. Homebrew rf circuit design ideas there is no such thing as a new idea. Va and Vb are the two input voltages and they are applied to the non inverting inputs of IC2 and IC1 respectively.
Electronic – BJT differential amplifier
Module History. Mr David Wiltshire. Both the written examination and practical assessment will require students to Analyse and distinguish the principles and operation of common electronic components. It will cover LOs: 1 - 3. The practical assessment will require the students to analyse and operate common electronic components. Learners are required during this module to complete theory examination — Closed Book. Item 2 - Exam EX Examination.
Is there any reason why the gain of a single ended input BJT differential amplifier, with one Rc, is a good approximation of the value of Adm for a common mode input. The simulation shows the frequency response for a single ended input and the circuit below shows th common mode input circuit. When you apply no AC signal to Q1's base, the shared emitter connection is still largely held at a small signal value and the gain of the right-side input to the left-side output is the same formula as the full differential mode amplifier gain: -.
Transistorised Differential Amplifier. The transistorised differential amplifier basically uses the emitter biased circuits which are identical in characteristics. Such two identical emitter biased circuits are. The differential amplifier can be obtained by using such two emitter biased circuits. This is achieved by connecting emitter E1 of Q1 to the emitter E2 of Q2.
Join Here! Already have an account? Log in. Design a BJT differential amplifier that provides two single-ended outputs at the collectors. Use a 2 -mA current source for biasing.
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