The present invention relates to electronics and more particularly to a high-speed differential preamplifier.
A preamplifier is an electronic component that is connected to a low-level signal source for providing suitable impedances and gain in an amplified signal that can be further processed without appreciable degradation in the signal-to-noise ratio. The signal source for a preamplifier can be a sensor that produces an electrical signal in response to a physical stimulus, such as light, heat, or pressure. In fiber-optic networks, the sensor can be a photodiode, which produces an electrical current in response to light. Other optical sensors include photoresistors, a device whose resistance decreases in response to a light stimulus, and a photocell, which produces an electric potential from an incidence of photons.
Preamplifiers are critical components in fiber-optic networks for controlling the noise sensitivity of optical front ends. One source of noise is known as xe2x80x9ccommon modexe2x80x9d noise, which manifests itself as a wandering direct current (DC) bias on the input signal. To achieve a high common mode noise rejection, it is desirable to employ a differential architecture, but differentially coupling the current signal produced by a photodiode to a preamplifier is difficult without the use of external components for alternating current (AC) coupling due to the DC reverse bias required by the photodiode.
One approach for coupling a photodiode to a preamplifier is depicted in FIG. 4. A circuit 400 comprises a differential amplifier 401 that has two input nodes 403 and 405 and two output nodes 407 and 409. A feedback resistor 411 having a resistance RF is coupled between output node 407 and input node 403, and another feedback resistor 413 having a resistance RF is coupled between output node 409 and input node 405. A photodiode 415 is coupled to have a DC reverse bias between VCC and input node 405. To provide a balanced circuit, a small capacitor 417 is coupled between the input node 403 and VCC.
In this architecture, the photodiode 415 produces a signal current IS that is single-ended DC coupled to the differential amplifier 401. The differential amplifier 401 produces an inverted voltage xe2x88x92DV at output node 407 and a non-inverted voltage DV at output node 409 based on a difference in the electrical signal at nodes 403 and 405. The small capacitor 417 absorbs such little charge that the inverted voltage xe2x88x92DV at output node 407 is also present at input node 403.
In the normal operation of the differential amplifier 401, input node 405 also has a voltage of xe2x88x92DV, which causes the potential difference across the feedback resistor 413 to be DVxe2x88x92DV=2 DV. By Ohm""s law, the potential difference across the feedback resistor 413 equals the product of the signal current IS flowing from photodiode 415 and the feedback resistance RF: ISxc3x97RF=2 DV, or DV=ISxc3x97RF/2. Accordingly, the single ended swing is RF/2 and the differential transimpendance gain is only RF. This gain limits the responsivity of the preamplifier in response to small input signals.
Attempting to improve the limited gain of the circuit 400 by increasing the values of the feedback resistors 411 and 41 or the capacitor 417 impairs other desirable characteristics of the circuit 400. For example, using either larger feedback resistors 411 and 41 or a larger capacitor 417 increases RC (resistancexc3x97capacitance) constant, which degrades the preamplifier""s bandwidth and dynamic range. Furthermore, this and many other approaches to increasing the preamplifier""s gain have been known to appreciably increase the power consumption and enlarge the integrated circuit real estate of preamplifier implementations.
A need therefore exists for a high-gain preamplifier, preferably without penalizing the preamplifier""s bandwidth or consuming significantly additional power or chip area. This need as well as others, is addressed by providing a current source at one amplifier input, which allows the signal current at the other input to flow through both feedback resistors, thereby doubling the gain. Bandwidth and dynamic range is preserved because the capacitance and resistance of the circuit remain the same.
Accordingly, one aspect pertains to a circuit for amplifying a signal from a sensor, which comprises a current source and a differential amplifier with one input coupled to the sensor and another input coupled to the current source. Feedback resistors may be coupled between the inputs and respective output, such that the gain of the amplifying circuit is approximately twice the feedback resistance. The current source may be configured to source a current greater than the current produced by the sensor and, in one implementation, include a field-effect transistor in saturation mode coupled to an emitter of a bipolar junction transistor. The amplifying circuit is part of an optical front, in which the sensor includes a photodiode and is coupled to a post amplifier.
Another aspect relates to a method for amplifying a signal from a sensor. A current generated by the sensor is received at a first node, while sinking, at a second node, a current greater than the current generated by the sensor. The signal is differentially amplified based on signals at the first node and the second node.
Still other aspects, features, and advantages are readily apparent from the following detailed description, simply by illustrating a number of particular embodiments and implementations, including the best mode contemplated for carrying out the present invention. The present invention is also capable of other and different embodiments, and its several details can be modified in various obvious respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive.