Amplifier with increased bandwidth and method thereof

[0001] 1. Field of the Invention

[0002] The present invention relates to an amplifier, and more particularly, to an amplifier with increased bandwidth by current injection.

[0003] 2. Description of the Prior Art

[0004] Operational amplifiers are one of the most widely used structures in modern day engineering. They can have many applications ranging from buffers and filters to analog-to-digital converters. Where high performance is required, such as a high speed, high resolution analog-to-digital converter, an operational amplifier having both high gain and high bandwidth is necessary.

[0005] Please refer to FIG. 1. FIG. 1 shows a conventional telescopic operational amplifier 10. It comprises a plurality of transistors M1-M9. Matched transistors M1 and M2 serve as an input stage 12 for respectively receiving a non-inverting input V_ip and an inverting input V_in, while transistors M3-M8 biased by voltages V_bn, V_bp1, V_bp2 act as a load stage 14 (i.e., an active load). The remaining transistor M9 is biased by voltage V_cmfb, and is the tail transistor acting as a current source for sinking a constant reference current I_Q, thereby steering the currents I₁ and I₂. Since transistors M5 and M6 are a matched pair and transistors M7 and M8 are a matched pair, currents I₁ and I₂ therefore have the same current value. If both transistors M1 and M2 are turned on, I₁ is equal to half I_Q, and I₁ is equal to I₂. As shown in FIG. 1, both the non-inverting output V_on and the inverting output V_op are coupled to an output capacitor C₀. A telescopic configuration is often a preferred configuration because telescopic operational amplifiers consume less power than other topologies. They also have the added advantage of high speed. The telescopic configuration is well known to those skilled in the art, and therefore is not described in detail here.

[0006] The frequency response of an operational amplifier is the response of the circuit in the frequency domain. In other words, if the circuit is given a sinusoidal input the response should be a sinusoidal output of the same frequency, but amplified by the open loop gain, A₀. This frequency response can be modeled as a low pass function.

[0007] $\begin{array}{cc}H\left(s\right)=\frac{A_0}{1+\frac{s}{{\omega }_p}}\mathrm{where}{\omega }_p\mathrm{is}\mathrm{the}\mathrm{dominate}\mathrm{pole}\mathrm{and}s=j\omega & \mathrm{Equation}\left(1\right)\end{array}$

[0008] For frequencies much greater than ω_p, i.e. ω>>ω_p, we can approximate the gain, A, to be:

[0009] $\begin{array}{cc}A\left(\mathrm{j\omega }\right)=\frac{A_0{\omega }_p}{\mathrm{j\omega }}& \mathrm{Equation}\left(2\right)\\ A\left(\mathrm{j\omega }\right)=\frac{A_0{\omega }_p}{\omega }& \mathrm{Equation}\left(3\right)\end{array}$
Therefore, putting this result back into equation (1), we can obtain the unity-gain bandwidth ω_u:
ω_u=A₀ω_p  Equation (4)

[0010] The unity gain bandwidth can also be written in terms of device transconductance, gm, and output resistance, r₀. Equation (4) then becomes:

[0011] $\begin{array}{cc}{\omega }_u=\frac{\mathrm{gm}\times r_0}{r_0{C}_0}=\frac{\mathrm{gm}}{C_{\mathrm{op}}+C_{\mathrm{load}}}& \mathrm{Equation}\left(5\right)\end{array}$

[0012] From equation (5) it can be seen that the unity gain bandwidth ω_u is a function of device transconductance gm, and total output capacitance C₀. The total output capacitance includes parasitic junction capacitance C_op, and capacitive load C_load driven by the operational amplifier 10. Therefore, the way to increase bandwidth is by increasing gm or by decreasing the output capacitance C₀. C_load cannot be decreased because it is determined by circuit specification and hence cannot be decreased for a specific application. Furthermore, the parasitic capacitance dominates the capacitive load, so decreasing C_load would only have a minimal effect. To increase gm would require a larger device, which then leads to more parasitic capacitance, as the device size cannot be increased without limits. This gain-bandwidth relation therefore creates an upper level bandwidth limit.

[0013] It is therefore one of the objectives of the claimed invention to provide an amplifier with increased bandwidth by current injection and a related method thereof.

[0014] It is therefore one of the objectives of the claimed invention to provide an amplifier. The amplifier with current injection has an increased bandwidth while maintaining a high gain.

[0015] It is therefore one of the objectives of the claimed invention to provide an amplifier. The amplifier does not require increased power consumption or circuit area.

[0016] Briefly described, the present invention discloses an amplifier. The amplifier comprises an input stage for receiving a first input signal; a load stage coupled to the input stage for generating a first output signal; a first current source coupled to the input stage for allowing a predetermined current to flow; and a second current source, coupled to the input stage, for injecting a first current into the input stage for the first output signal.

[0017] In addition, a method for increasing the bandwidth of an amplifier is disclosed. The method comprises providing the amplifier with an input stage for receiving a first input signal, a load stage coupled to the input stage for outputting a first output signal, and a predetermined current source coupled to the input stage for allowing a fixed current to flow; and providing a first current and injecting the first current into the input stage for outputting the first output signal.

[0018] These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

[0022] Please refer to FIG. 2. FIG. 2 is a diagram of a telescopic operational amplifier 20 with current injection according to a first embodiment of the present invention, for increasing the bandwidth while maintaining a high gain. The telescopic operational amplifier 20 includes an input stage 22, which has two transistors M1 and M2, and a load stage 24, which has a plurality of transistors M31, M41, M51, M61, M71, M81. The transistor M9 acts as a current source for sinking a constant reference current I_Q, thereby steering the currents Ia-Id. If both transistors M1 and M2 are turned on, the collective currents I_a and I_c are equal to the current I₁ shown in FIG. 1, and the collective currents I_b and I_d are equal to the current I₂ shown in FIG. 1. Please note that the circuit architecture of the input stage 22 and load stage 24 is substantially the same as that of the input stage 12 and load stage 14 shown in FIG. 1. The amplifier 20 of the present invention further comprises extra current sources 26 and 28.

[0023] As shown in FIG. 2, a series of electrically connected transistors M72, M52 and M32 are coupled to the drain of the transistor M1, and a series of electrically connected transistors M82, M62 and M42 are coupled to the drain of the transistor M2, for respectively injecting a current I_c and a current I_d into the input stage 22. The transistors M72 and M82 are biased by a voltage V_bp1, M52 and M62 are biased by a voltage V_bp2, and M32 and M42 are biased by a voltage V_bp3. Additionally, an inverting output V_on coupled to an output capacitor C₀ is coupled to the drain of the transistor M31 and an non-inverting output V_op coupled to an output capacitor C₀ is coupled to the drain of the transistor M41. In an embodiment, transistors M32, M42, M51, M52, M61, M62, M71, M72, M81, M82 are PMOS transistors, and transistors M1, M2, M9, M31, M41 are NMOS transistors. Please note this amplifier configuration is merely an embodiment of the present invention and is not meant to be a limitation.

[0024] In an embodiment, both of the current sources 26, and 28 are p-type current sources. Since the transistor M9 still sinks the reference current I_Q, the original currents I₁, I₂ shown in FIG. 1 are changed due to the injected currents I_C, I_d. In order to maintain the same bias condition, the size of the devices is scaled, according to the change of current of each device. Assume that I_a is set to k*I₁. Therefore, the channel aspect ratio (W/L) is adjusted accordingly. In other words, for transistor M71 as compared to transistor M7 in FIG. 1, the channel aspect ratio is:

[0025] $\begin{array}{cc}{\left(\frac{W}{L}\right)}_{M71}=k*{\left(\frac{W}{L}\right)}_{M7}\mathrm{where}0<k<1& \mathrm{Equation}\left(6\right)\end{array}$

[0026] Similarly, for transistor M81 as compared to transistor M8 in FIG. 1:

[0027] $\begin{array}{cc}{\left(\frac{W}{L}\right)}_{M81}=\left(1-k\right)*{\left(\frac{W}{L}\right)}_{M8}\mathrm{where}0<k<1& \mathrm{Equation}\left(7\right)\end{array}$
The frequency response for the operational amplifier 20 can still be modeled as a low-pass function as shown by equation (1). The dominate pole ω_p, however, is now:

[0028] $\begin{array}{cc}{\omega }_p=\frac{1}{r_0{C}_0}=\frac{1}{r_0\left({\mathrm{kC}}_{\mathrm{op}}+C_{\mathrm{load}}\right)}& \mathrm{Equation}\left(8\right)\end{array}$

[0029] This is because the device size has decreased by a factor of k, where 0<k<1, and therefore the parasitic capacitance C_op has also decreased by a factor of k.

Putting this result in the equation for unity-gain bandwidth ω_u it can be shown that:

[0030] $\begin{array}{cc}{\omega }_u=\frac{\mathrm{gm}\times r_0}{r_0{C}_0}=\frac{\mathrm{gm}}{r_0\left({\mathrm{kC}}_{\mathrm{op}}+C_{\mathrm{load}}\right)}& \mathrm{Equation}\left(9\right)\end{array}$
Due to the smaller device parasitic capacitance C_OP, the unity-gain bandwidth ω_u, compared with the prior art unity-gain bandwidth, has been increased by a factor:

[0031] (C_op+C_load)/(kC_op+C_load). Please note that the circuit size of the operational amplifier 20 is substantially the same as that of the operational amplifier 10 shown in FIG. 1.

[0032] Please refer to FIG. 3. FIG. 3 is a diagram of a telescopic operational amplifier 30, with gain boosting and current injection, according to a second embodiment of the present invention, for increasing the bandwidth while also increasing the gain. The telescopic operational amplifier 30 is similar to the above telescopic operational amplifier 20. The lengthy description of the same components, i.e., the transistor M9, the input stage 22 and the current sources 26, 28, is not repeated. As shown in FIG. 3, the key difference is that the telescopic operational amplifier 30 has a load stage 32 including a plurality of amplifiers 33, 34, 35, 36 used to enable the gain boosting. Since the gain boosting is well known to those skilled in this art, further description is omitted for brevity. Compared with the telescopic operational amplifier 20, the telescopic operational amplifier 30 has an increased DC gain A₀ due to the gain boosting. In addition, with the currents injected into the input stage 22 by the current sources 26 and 28, the bandwidth of the operational amplifier 30 is thereby increased.

[0033] Please note that, in the above-mentioned embodiments, the current injection is applied to a differential pair. However, both of the current injection configurations shown in FIG. 2 and FIG. 3 are just exemplary embodiments, and are not meant to be limitations.

[0034] The embodiments of the present invention have an increased bandwidth without sacrificing gain. The circuit area and power consumed are still approximately the same but the parasitic capacitance has been decreased. The current injection technique can also be used in a gain boosted operational amplifier, therefore allowing both the bandwidth and the gain to be increased at the same time.

[0035] Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.